GUI - overhaul shortcut functionality

* All shortcuts are now centralised in the menubar as QActions
* There are now different shortcut modes - Emacs Live, Windows, Mac and User
* User mode can be modified by a ~/.sonic-pi/config/keyboard-shortcuts.ini file

The ini file could look like this (; are comments)

;Base=Emacs
;Base can be one of Emacs, Win, Mac or None
;None is the default

;shortcuts should be in the form of Modifier+Key
;Available modifiers are:
;(Where Meta is Alt on Windows and Linux, and Command on Mac)

;  * Ctrl
;  * Shift
;  * Meta
;  * ShiftCtrl
;  * ShiftMeta
;  * Alt

Run=Meta+R
;Stop=
;Record=
;Save=
;Load=
;Align=
;Comment=
;Transpose=
;ShiftUp=
;ShiftDown=
;ContextualDocs=
;TextZoomIn=
;TextZoomOut=
;Scope=
;CycleThemes=
;Info=
;Help=
;Prefs=
;TabPrev=
;TabNext=
;Tab1=
;Tab2=
;Tab3=
;Tab4=
;Tab5=
;Tab6=
;Tab7=
;Tab8=
;Tab9=
;Tab0=
;Link=
;TapTempo=
;FocusEditor=
;FocusLogs=
;FocusContext=
;FocusCues=
;FocusPrefs=
;FocusHelpListing=
;FocusHelpDetails=
;FocusErrors=
;FocusBPMScrubber=
;FocusTimeWarpScrubber=
;ShowButtons=
;ShowCueLog=
;ShowLog=
;SetMark=
;logZoomIn=
;logZoomOut=
;Down=
;Up=
;UpTen=
;DownTen=
;CutToEnd=
;Copy=
;Cut=
;Paste=
;Right=
;Left=
;DeleteForward=
;DeleteBackward=
;LineStart=
;LineEnd=
;DocStart=
;DocEnd=
;WordRight=
;WordLeft=
;CenterVertically=
;Undo=
;Redo=
;SelectAll=
;DeleteWordRight=
;UpcaseWord=
;DowncaseWord=
;Fullscreen=