Added set/getTextSelectable() method to GafferUI.Label.

By default the text is not selectable, as before.
Added Dialogue set/getModal() methods.

These allow the management of a modal event loop of the same sort used by the existing waitForButton() method. This new lower level functionality can be used by dialogues which wish to stay modal until some event other than a button press has occurred.
Added new icons for use in OpDialogue.
Added background operation mode to OpDialogue.

This executes the op on a background thread, allowing the UI to display an animated busy signal as the op does its thing. Also added new postExecuteBehaviours to allow the display of the result and a success/failure icon, before allowing the user to optionally run the op again.
Updated the BrowserEditor and op app to execute ops in a background thread.