New skin compatibility toolkit

[mazkolain-zz]

Experiment with a new skin toolkit that would improve the current xbmc-skinutils in use. This toolkit would make Spotimc more friendly to the XBMC's official repo rules, and would remove many quirks of the current one:

• No need to patch current skin
• No need to copy skin to userdata
• No need to perform userdata / main skin version checks
• No need to reload current skin on startup and shutdown

It would operate as a skin preprocessor, with two clear parts:

• An include substitution engine that would replace statements with the actual markup behind them.
• A font compatibility table: That would enumerate the stantard (in terms of size) fonts for each skin.
• A variable substitution engine (to be used with the standard font table).

As a fallback, this compatibility table could be built on the fly reading skin's fonts xml, but my first impression is that it would be somewhat error prone.

Caching would be a gread addition too, by using all the involved file timestamps.