Add support for jpg icons; only replace shortcuts where added by UWPHook

[DragRedSim]

Icons: Steam does support JPG images, it just doesn't expose them as an option in its own finder for selecting an icon for a shortcut. Fixes XGP game ReCore, which only has one, non-square PNG asset; all other logos are JPG.

Shortcut maintenance: only replace a shortcut in the config.vdf where the name matches, and the EXE in the config.vdf is the UWPHook EXE being used to add the icon. This avoids cases where the same game may be installed in multiple ways; it also plays nicer with other launcher tools.