Avoid possible clash of Qt modules with USD

[kwokcb]

Changes

Delay loading Qt modules as they appear to clash with USD imports.
Workaround for #103.

Details

This appears to be the plugin load location:

plugin_manager = qx_plugin.QuiltiXPluginManager("QuiltiX")
plugin_manager.load_plugins_from_environment_variable()
plugin_manager.add_hookspecs(qx_plugin.QuiltixHookspecs)

which will cause the Qt module imports in the plugin.

Usd appears to be imported afterwards:

from pxr import Usd, UsdShade

By loading Qt when after_ui_init occurs:

plugin_manager.hook.after_ui_init(editor=self)

this seems to make this "run" at least.

Test

Sample JSON plugin will load.