Some app meta-info updates

[bjorn]

• Switch from deprecated "desktop" to "desktop-application".

• Add captions to screenshots, remove unnecessary size attributes.

• Add branding colors.

• Don't use "requires" for pointing/keyboard because it will cause GNOME Software to claim that Tiled does not support keyboard/mouse at all.

• Removed "touch" otherwise GNOME Software will claim that Tiled "works on phones, tablets and desktops".

• Add display_length element with expected value to tell GNOME Software that Tiled is a desktop application. Unfortunately, now it will state that "Tiled will not work properly on this device", but leaving this attribute out will make it say "Desktop Support Unknown"...

TODO:

• Look into updating the icon and screenshots.