Add ability to disable controls during AJAX.

QAjaxAction and QAjaxControlAction will now accept an optional
ToggleEnableControls parameter of either a single QControl or an array
of QControls to disable during the AJAX action processing. The
control(s) will be enabled again after the AJAX action completes.
Tweaks for toggling controls during AJAX actions
Merge pull request #28 from romcart/master

Feature to disable controls during AJAX actions