Add "hidden" tag to gamelist, add RunOnX functions.

- New tag "hidden" in gamelist.xml to hide e.g. emulator BIOS files from
the game list.
- New settings in es_settings.cfg: "RunOnFolderSelected" and
"RunOnGameSelected" can be used to run a process when a folder/game has
been selected in the GUI. "RunOnFolderSelected" can use the %PATH% tag
(folder path), "RunOnGameSelected" can use %ROM%, %BASENAME% and
%ROM_RAW%.