Remove no more used `menu/interface-config`.
Add `menu/highlighting`, `menu/vr-options`, `options/system-time`,
`options/gmt`, `options/local-aircraft-time`.
Correct tip text from "Hold [Shift] while looking around to move the
view up, down, left or right" to "Hold Ctrl ...".
Corrections for Polish nonQt translations.
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/>.
