osxbundle: activate Game Mode with App bundle

mpv isn't really a game and categorising our App bundle as game seems
counterintuitive. though as a video player it does have some
similarities, possibly needing considerably CPU and GPU resources and
low presentation latencies.

the macOS Game Mode promises exactly that, it gives App the highest
priority access to CPU and GPU resources and lowers usage for background
tasks.

sadly the Game Mode can only be activated by categorising the App as a
game and that is only possible with an App bundle and not as a command
line tool.

TOOLS/osxbundle/mpv.app/Contents/Info.plist

@@ -188,6 +188,8 @@
     <string>${VERSION}</string>
     <key>NSHighResolutionCapable</key>
     <true/>
+    <key>LSApplicationCategoryType</key>
+    <string>public.app-category.games</string>
     <key>LSEnvironment</key>
     <dict>
       <key>MallocNanoZone</key>