Imgui: make imgui emulator settings accessible with hotkey

[rainmakerv3]

Add a hotkey to make the imgui settings dialog (previously only accessible from big picture mode) accessible ingame. The config active config file in use (global of the running game's game-specific config if it exists) will be loaded.

Some other changes added in:

1. Puts all big picture stuff in the same folder just to make them clearly separated from files used by the core emulator
2. Make imgui settings window a class to better manage private methods/variables
3. changed the profile selection screen so that game-specific configs can be deleted from this interface, and so it doesn't look so similar to the big picture mode's screen for launching games
4. slight cosmetic changes

New profile selection screen: