Add Simplified Chinese startup tips (disabled for now)

The tips are not enabled in Translations/locale.xml yet, because the
font currently used to display them in FlightGear's splash screen
doesn't have the required glyphs (see [1] for more details). Better
display English tips than empty rectangles on the startup screen.

Merging this in disabled state now has a few advantages compared to
waiting for the font problem to be solved:
  - avoid that someone ignoring the merge request[1] duplicates the
    translation work;
  - allow easy updating of these tips;
  - make sure they aren't forgotten when FG's localization files are
    converted to XLIFF format.

Ship an empty Translations/zh_CN/options.xml, otherwise
'fgfs --language=zh_CN --help --verbose' won't work at all.

[1] https://sourceforge.net/p/flightgear/fgdata/merge-requests/92/

Translations/locale.xml

@@ -106,6 +106,22 @@
 		</strings>
 	</locale>
 
+        <locale>
+		<lang type="string">zh</lang>
+		<lang type="string">zh_CN</lang>
+
+		<strings>
+			<options>Translations/zh_CN/options.xml</options>
+			<!-- Chinese startup tips disabled for now because the
+			     font used to display them in FlightGear's splash
+			     screen doesn't have the required glyphs
+			     (2017-06-25). cf.
+			     https://sourceforge.net/p/flightgear/fgdata/merge-requests/92/
+			-->
+			<!-- <tips>Translations/zh_CN/tips.xml</tips> -->
+		</strings>
+        </locale>
+
 	<locale>
 		<!--  test character encoding using - -language=TEST -->
 		<lang type="string">TEST</lang>

Translations/zh_CN/options.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+<!-- FlightGear options: Simplified Chinese language resource -->
+
+<PropertyList>
+</PropertyList>

Translations/zh_CN/tips.xml

@@ -0,0 +1,27 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+<!-- FlightGear startup tips: Simplified Chinese language resource -->
+
+<!-- Style tips: Aim to keep to a single phrase/sentence which is not
+                 terminated with a . -->
+<PropertyList>
+	<tip n="0">在按住鼠标右键时滑动鼠标以观察四周</tip>
+	<tip n="1">在观察四周时按住 [Shift] 来将视角向上、下、左、右移动</tip>
+	<tip n="2">按下 a/A 来调整模拟器速度（倍速）</tip>
+	<tip n="3">按下 x/X 来放大或缩小</tip>
+	<tip n="4">从 Radios 对话框可查询附近ATC频率</tip>
+	<tip n="5">按下 v 来更换视角, 按下 Ctrl-V 可回到驾驶舱视角</tip>
+	<tip n="6">按下 F3 可以截图</tip>
+	<tip n="7">欢迎加入FlightGear社区，网址为 http://forum.flightgear.org</tip>
+	<tip n="8">按下 h 来启用抬头显示器（HUD）</tip>
+	<tip n="9">按下 F10 来隐藏或显示菜单栏</tip>
+	<tip n="10">帧率太低？ - 试试禁用AI交通</tip>
+	<tip n="11">选择 Environment → Weather 在不同天气条件下飞行</tip>
+	<tip n="12">按下F12可轻易更换无线频率</tip>
+	<tip n="13">在按住鼠标右键时滑动鼠标以观察四周</tip>
+	<tip n="14">选择 Multiplayer → Multiplayer Settings 来加入联机飞行</tip>
+	<tip n="15">不知如何启动引擎？ - 查看 Help → Aircraft Checklists</tip>
+	<tip n="16">在按住鼠标右键与中键时滑动鼠标以移动观察点</tip>
+	<tip n="17">按下 / 来浏览FlightGear的内部属性</tip>
+	<tip n="18">按下 Tab 后使用鼠标来移动观察点，并使用滚轮放大或缩小</tip>
+</PropertyList>