Merge pull request #1128 from andrewkaufman/imageQuickFixes

GafferImage Quick Fixes
Deferred creation of plug presets submenu.

In case someone does something like registering a preset which makes calls to an outrageously slow database somewhere.
Merge pull request #1129 from johnhaddon/deferredPresetsMenu

Deferred creation of plug presets submenu.
Added a Refresh button to ImageReaders.