Explore database power

[jgeumlek]

If I was a better person, I'd actually make this into separate pull requests, but instead I'll just leave this here. I don't expect it to be merged, what with the understandable stagnation and all. If desired, I'll gladly cleanup and submit a proper pull request for any of these features.

I've been hacking a version of EmulationStation to better serve my personal needs. I've built off of the gamelistdb branch and used the database to add some features, (but also: made menus cycle around at the top and bottom, and greatly restructured the menus to make the "start" button start games). The changes have been for my own desires, and have not been tested outside of a linux desktop environment.

Major database features/changes:

• Database is trusted (database updated manually, start up doesn't scan file system repeatedly)
• New file type: Filter. Analogous to a Folder, but uses an arbitrary SQL expression to determine children. Plus an adequate-but-not-great UI for creating a filter.
• Arbitrary "tags" can be assigned/removed on files, and tags can be used in filters. (database has a separate table for tags) [UI for adding/removing tags is pretty bad at the moment]
• Sort options made more friendly/flexible using the power of SQL. (Still hardcoded options, though)
• Systems can have a filter specified to limit shown/available files, such as hiding unwanted ones. Meta systems can be created with no files of their own, instead they show files from all systems.