Ignore mouse events that didn't start with a click.

Ignore mouse events that didn't start with a click.

On OS X GTK, when you click in a pterm that wasn't the active window,
the first click activates it but is swallowed by the windowing system
- but a subsequent tiny drag can still be taken as part of a selection
action, making it difficult to activate the window in order to paste
into it.

Fixed by ignoring mouse drags when the terminal.c mouse state was
NO_SELECTION; if we've seen one prior click then it should be
ABOUT_TO, or DRAGGING if we saw a double or triple click.