[TIMOB-18868] Do not overwrite Info.plist keys set by user

This ticket describes the changes in the original PR ([#6408](https://github.com/appcelerator/titanium_mobile/pull/6408)) for TIMOB-17993. That issue requested a way to set CFBundleVersion and CFBundleShortVersionString separately without asking for a specific solution. The PR resolved this by no longer overwriting these two, *but also other keys* set in tiapp.xml/ios/plist or a custom Info.plist. So the request of this new ticket is for the build process to not overwrite keys set in tiapp.xml/ios/plist or a custom Info.plist.

Current situation

1. Set some defaults (UIStatusBarStyle, CFBundleIdentifier..) 2. Merge keys in custom Info.plist in project root 3. Merge keys in tiapp.xml/ios/plist 4. Set CFBundleVersion, CFBundleShortVersionString 5. Set CFBundleIconFiles, UILaunchImages, UIAppFonts

New situation

1. Set some defaults (UIStatusBarStyle, CFBundleIdentifier..) 2. Merge keys in custom Info.plist in project root 3. Merge keys in tiapp.xml/ios/plist 4. Set CFBundleVersion, CFBundleShortVersionString *only if not set by 2 or 3* (fixed by [#6818](https://github.com/appcelerator/titanium_mobile/pull/6818) for TIMOB-17993) 5. Set CFBundleIconFiles, UILaunchImages, UIAppFonts *only if not set by 2 or 3* (fixed by [#6408](https://github.com/appcelerator/titanium_mobile/pull/6408) for this issue)

PR

The PR on master for this issue, but also including that of TIMOB-17993: https://github.com/appcelerator/titanium_mobile/pull/6408

Note

For UILaunchImages [417f378430ba85908135c41a2806a5d49f271ae4](https://github.com/appcelerator/titanium_mobile/commit/417f378430ba85908135c41a2806a5d49f271ae4#diff-f430483233aa01af5b10df8390f9635dR2083) already tried to not overwrite values set via tiapp.xml/ios/plist or Info.plist but because it was checking the merged plist variable, it also skipped when the build directory already was generated, requiring the project to be cleaned every time you add a launch image. My PR fixes this.