Support changing icons for non-Steam shortcuts

[Jozen Blue Martinez]

Possible implementation:

1. User boops a non-Steam icon.
2. Shortcut is selected from popup.
3. Save icon as Steam/userdata/XXXXXXXX/config/grid/APPID_icon.ext (path can actually be anywhere, but SRM and other programs unofficially save in this path already)
4. Add or edit the icon key in shortcuts.vdf to point to the saved icon location.

[Jozen Blue Martinez]

Progress done in nonsteam-icons. Requires more review/testing due to the possibility of severely messing up shortcuts.vdf.

[mattsxp]

Not working on my end with latest version