Blender Git Loki

Fix for #36387, User Preferences "Addons" panel bogs down the whole interface.
The addons panel draw function calls addon_utils.modules() which in turn retrieves a list of fake modules from the script paths every time. This can become costly when network paths are included for
addons. Solution is to put the scanning process into a dedicated "refresh" function and disable it in frequently called draw and filter functions, i.e. in these cases the cached addons_fake_modules list
will be used instead.

Note that this may lead to invalid addon lists if script paths are changed (which is not working 100% without restart anyway according to Campbell). For this there is now a "Refresh" operator button in
the addons preferences. If necessary and feasible such forced refreshes can be added later too.