Add option to manipulate UI save states from Lua

[notdodgeball]

Describe the bug

As of now the lua functions have a different mechanism (from the UI menu options) to create and load save states.

Expected behavior

I could just use
PCSX.loadSaveState( 1 )
PCSX.createSaveState( 1 )
and load/save from the UI slot 1.

Also, a option to indicate a global slot would be appreciated.