Translations: rename Translations/en to Translations/default

This is mainly to make it clear that the "translation" in (after this
commit) Translations/default, a priori corresponding to en_US, has a
special status: it acts as the source describing the set of translatable
strings, each with its id (an XML tag name) and numeric index (a
PropertyList index, often 0 but not always).

In the upcoming i18n infrastructure, the files in Translations/default
will be used to:
  - detect all translatable strings (~ string extraction);
  - detect obsolete translated strings (those in XLIFF files but not in
    Translations/default anymore);

This will allow a script to update the XLIFF files containing real
translations, i.e., update the source strings and possibly remove
obsolete entries.

So, the files in Translations/default have a special "master" status in
this process, and are technically quite different from those in
Translations/de, Translations/es, Translations/fr, etc. The latter XML
files (those not in Translations/default) will be replaced with their
XLIFF equivalent, by the way (conversion script already working).

See discussion at
<https://sourceforge.net/p/flightgear/mailman/message/35910579/>.

Translations/locale.xml

@@ -13,11 +13,11 @@
 		<lang type="string">default</lang>
 
 		<strings>
-			<options>Translations/en/options.xml</options>
-			<menu>Translations/en/menu.xml</menu>
-			<sys>Translations/en/sys.xml</sys>
-			<atc>Translations/en/atc.xml</atc>
-			<tips>Translations/en/tips.xml</tips>
+			<options>Translations/default/options.xml</options>
+			<menu>Translations/default/menu.xml</menu>
+			<sys>Translations/default/sys.xml</sys>
+			<atc>Translations/default/atc.xml</atc>
+			<tips>Translations/default/tips.xml</tips>
 		</strings>
 	</locale>
 
@@ -31,7 +31,7 @@
 			<options>Translations/es/options.xml</options>
 			<menu>Translations/es/menu.xml</menu>
 			<sys>Translations/es/sys.xml</sys>
-			<atc>Translations/en/atc.xml</atc>
+			<atc>Translations/default/atc.xml</atc>
 			<tips>Translations/es/tips.xml</tips>
 		</strings>
 	</locale>
@@ -46,7 +46,7 @@
 			<options>Translations/de/options.xml</options>
 			<menu>Translations/de/menu.xml</menu>
 			<sys>Translations/de/sys.xml</sys>
-			<atc>Translations/en/atc.xml</atc>
+			<atc>Translations/default/atc.xml</atc>
 		</strings>
 	</locale>
 
@@ -61,7 +61,7 @@
 			<options>Translations/fr/options.xml</options>
 			<menu>Translations/fr/menu.xml</menu>
 			<sys>Translations/fr/sys.xml</sys>
-			<atc>Translations/en/atc.xml</atc>
+			<atc>Translations/default/atc.xml</atc>
 			<tips>Translations/fr/tips.xml</tips>
 		</strings>
 	</locale>
@@ -76,7 +76,7 @@
 			<options>Translations/it/options.xml</options>
 			<menu>Translations/it/menu.xml</menu>
 			<sys>Translations/it/sys.xml</sys>
-			<atc>Translations/en/atc.xml</atc>
+			<atc>Translations/default/atc.xml</atc>
 			<tips>Translations/it/tips.xml</tips>
 		</strings>
 	</locale>
@@ -90,7 +90,7 @@
 			<options>Translations/nl/options.xml</options>
 			<menu>Translations/nl/menu.xml</menu>
 			<sys>Translations/nl/sys.xml</sys>
-			<atc>Translations/en/atc.xml</atc>
+			<atc>Translations/default/atc.xml</atc>
 		</strings>
 	</locale>
 
@@ -102,7 +102,7 @@
 			<options>Translations/pl/options.xml</options>
 			<menu>Translations/pl/menu.xml</menu>
 			<sys>Translations/pl/sys.xml</sys>
-			<atc>Translations/en/atc.xml</atc>
+			<atc>Translations/default/atc.xml</atc>
 		</strings>
 	</locale>
 
@@ -139,7 +139,7 @@
 			<options>Translations/cs/options.xml</options>
 			<menu>Translations/cs/menu.xml</menu>
 			<sys>Translations/cs/sys.xml</sys>
-			<atc>Translations/en/atc.xml</atc>
+			<atc>Translations/default/atc.xml</atc>
 		</strings>
 	</locale>
 
@@ -150,7 +150,7 @@
 			<options>Translations/da/options.xml</options>
 			<menu>Translations/da/menu.xml</menu>
 			<sys>Translations/da/sys.xml</sys>
-			<atc>Translations/en/atc.xml</atc>
+			<atc>Translations/default/atc.xml</atc>
 		</strings>
 	</locale>
 
@@ -190,7 +190,7 @@
 			<options>Translations/pt/options.xml</options>
 			<menu>Translations/pt/menu.xml</menu>
 			<sys>Translations/pt/sys.xml</sys>
-			<atc>Translations/en/atc.xml</atc>
+			<atc>Translations/default/atc.xml</atc>
 		</strings>
 	</locale>
     <!--