Fix-up mwcore split: Split out MWDialogTool

Because everything was broken.

Follows-up I6374854eaa1 which split up mwcore into smaller parts but
left MWDialogTool still as one file part of mwcore. Since the various
ve.dm classes are now part of the other modules that depend on mwcore,
the MWDialogTool classes were pointing at classes that don't exist yet
(e.g. ".modelClasses = [ ve.dm.MWBlockImageNode ];").

Change-Id: Ic05b173d730a8a0f6539fdcd818aaf674d2a4019