Abstract Maya UI funtionality into ui_tools module

[eestrada]

This is part of a general refactoring of code to improve maintainability and portability of the code base.

Pretty much all of this has been done for Houdini already. There is only one direct call to the hou.ui module remaining in the hou_asset_mgr module. Ultimately there should be no calls to the hou.ui module (or any application specific UI module) anywhere in the code other than in the ui_tools module.