Make game tags / metadata available with %tag% style placeholders

[fraganator]

This idea is following on from my previous suggestion in issue #2086.

It'd be very useful if a game's tags and metadata were exposed through the use of %tag% style placeholders.

For example if I'm adding an additional app for a game (perhaps a second emulator, or a pre-processing routine), I'd like to be able to use a tag placeholder like %rom_file% to refer to the current game's ROM file and path. These placeholder tags could also be used when setting up emulator command lines.