GUI fixes: shortcuts, mouse wheel zoom, save/restore settings

* Move Toolbar item shortcuts to application-wide QShortcuts so they work when toolbar is hidden, fixes https://github.com/samaaron/sonic-pi/issues/130
* Enable mouse-wheel zoom for text font sizes via Ctrl+mouse wheel.
* Save/restore font size per workspace to QSettings
* Save/restore window state (dock sizes and states, including help pane) to QSettings.