macOS: Refactor app icon into property
This MR refactors the App icon handling into a method in VLCApplication
for a new property vlcAppIconImage
. This is subsequently used to access
the current icon when the app starts up and for the about window,
possibly in other places in the future too.
The reason for not just using NSApplication applicationIconImage
is that
it is scaled to a lower resolution and would look blurry in the About panel.