Improve user experience for apps in disk images

[probonopd]

This is the current helloSystem user experience:

This is the user experience users know from elsewhere:

Thanks @Anvith00 for providing the screenshots.

To achieve this effect,

1. A symlink to the /Applications folder must be inside the disk image
2. The properties of the window (including icon size and position) must be stored (Details)
3. A background image must be shipped. The background image contains the arrow
4. The file manager must know how to use the background image
5. For custom icons for the volume (mounted disk on the desktop), support .VolumeIcon.icns

We could do 1. immediately and easily. However, some people may want to put applications into /Applications whereas others might want to put them into ~/Applications, in folders within those folders, or in entirely different places. I always found it a bit limiting to suggest that all applications need to to to /Applications and never really put them there, even on the Mac. One could argue, though, that we should still put in the symlink as users are not forced to use it.

As for the other points, some engineering would be needed. 2. is currently not implemented all the way, and 3. is not implemented yet but should not be complicated. For 4. see #127. 5. has already been done.

To test 5.:

wget --trust-server-names "https://download.mozilla.org/?product=firefox-latest-ssl&os=osx&lang=en-US"
./darling-dmg ./Firefox*.dmg /mnt
FreeBSD% ./darling-dmg Firefox\ 106.0.dmg /media/Firefox 

Will in fact show the correct icon.

So to make this work, someone would need to implement 2. @moochris has done great initial work but unfortunately is no longer active in the project, so it was never finished so far. Vounteers?

References:

• Spatial file manager hello#79
• Support custom window backgrounds #127