Added support for setting the AppStore icon on iOS

[livecodepanos]

[livecodepanos]

Changes:

In group "Icons":

the ... button for icon selection has the following code:

on mouseUp
   chooseImageFileWithDimension "AppStore Icon", "1024,1024"
end mouseUp

Its tooltip is set accordingly.

The X button for clearing the icon has the code:

on mouseUp
   clearSetting "AppStore Icon"
end mouseUp

[livecodeali]

@livecode-vulcan review ok d83e185

[livecode-vulcan]

💙 review by @livecodeali ok d83e185