"Next Slot" control mapping

-added "Next Slot" to control mapping menu
-added VIRTKEY_NEXT_SLOT -> incremented VIRTKEY_TOGGLE_FULLSCREEN
-added SaveState::NextSlot(); as replacement for code in
WndMainWindow
-added entry in EmuScreen using SaveState::NextSlot();
Win32: Add option to disable the Windows key.
Add a comment for the keyboard hook.
Let's be consistent with our spacing and if's.
Win32: Add the option to the Game Settings menu, too, and add a separator in NewUI.
When loading a savestate with more ram, realloc.

Also, I'm not sure the extra ram is being handled correctly, I think it's
not meant to be exposed to games except homebrew, not sure of the exact
method...
Revert the gp struct member in mpegs for now.

Unlikely to be used, even though seen in tests, and could be
causing #5045 (just a guess.)
Merge pull request #5050 from unknownbrackets/mem-fix

Fix savestates with non equal pspmodel, revert mpeg->gp
Merge pull request #5040 from zack-vii/NextSlotKey

"Next Slot" control mapping
Fix a few range checks in scePsmf. See #5051
Merge pull request #5043 from thedax/win32IgnoreWindowsKeyOption

Win32: Add an option to disable the Windows key when the emulator is the active window.
Merge remote-tracking branch 'upstream/master' into fps2