switch to image asset for icons and spashscreens

contrib/mobile/CMakeLists.txt

contrib/mobile/iOS/Onelab.xcodeproj/project.pbxproj

...S/Onelab/Images.xcassets/AppIcon.appiconset/Contents.json

...elab/Images.xcassets/AppIcon.appiconset/icon_app_ipad.png

...ages.xcassets/AppIcon.appiconset/icon_app_ipad_retina.png

...es.xcassets/AppIcon.appiconset/icon_app_iphone_retina.png

...lab/Images.xcassets/LaunchImage.launchimage/Contents.json

...mages.xcassets/LaunchImage.launchimage/splash1024x768.png

...ages.xcassets/LaunchImage.launchimage/splash1536x2048.png

...ages.xcassets/LaunchImage.launchimage/splash2048x1536.png

...mages.xcassets/LaunchImage.launchimage/splash640x1136.png

...Images.xcassets/LaunchImage.launchimage/splash640x960.png

...mages.xcassets/LaunchImage.launchimage/splash768x1024.png