Optimize widgets (but better)

[thecraftianman]

Based on (and supersedes) #915
Closes #2902

In addition to the changes made in the original PR, the following improvements have been made:

• The filter function now uses a whitelist table instead of a function
• The hooks handling widget processing are only added when widgets are in use
• widgets.RenderMe is no longer necessary (its logic has been optimized and moved into the rendering hook directly) and has been stubbed out. I'm not sure if any addons that interact with this function even exist, so any suggestions on whether it should be kept functional, removed entirely, or kept stubbed out would be appreciated
• All widget entities have had some amount of extra optimization done (mostly just reusing variables more often)

The widget system still won't be particularly performant, but it runs at least decently faster now, and manually removing the system entirely should no longer be necessary for a performance boost.

[robotboy655]

Found this regression when testing for 1 second. (Just the widget.lua file)

The blue box is not supposed to appear.

[thecraftianman]

There's a corresponding change in widgets_axis.lua (if I remember right) that fixes the issue with this particular widget. This should only affect this one widget because of the way that it attempted to prevent itself from being drawn before when widgets.RenderMe was still functional.

[robotboy655]

Do you not see the potential issues this can create with 3rd party addon content?