Add ctrl-V shortcut for paste in edit controls

This commit implements ctrl-V as a shortcut for paste, but only in edit controls. Globally ctrl-V can be mapped however the user wishes using the usual keyboard mapping file.

I've tested on both Windows and OSX, though for now I have not changed the hardcoded ctrl-V handler in xbmc/windowing/WinEventsSDL.cpp. I've put the code in for Linux, though at the moment Linux does not handle ACTION_PASTE.