Frame Level and Strata control!

[Goranaws]

I needed this feature for some UI modifications. I attempted to stay as true to your coding style as possible. This implementation only displays on the option menus that include the pre-built AdvancedPanel. Options menu access will need to adjusted, as I assume you have a dedicated options panel in mind for frame layering and similar features.

And, you had mentioned you were going to include this, eventually, so here is half the work done. :-)

[Tuller]

Merged but still a few bits to do

• [x] Dropping "DIALOG", "FULLSCREEN", "FULLSCREEN_DIALOG", "TOOLTIP". I don't think we should use those stratas for things.
• [x] Localize stuff. A few of them (LOW, HIGH, BACKGROUND) have values already, but not MEDIUM
• [ ] Update drag frames when the strata is updated. That's DRAG_FRAME_LEVELS in overlay/ui.lua. The levels should likely be DRAG_FRAME_LEVELS[owner:GetDisplayLayer()] + owner:GetDisplayLevel(). Probably should use callbacks to notify the UI layer when things change.
• [x] Update the API to use DisplayLayer for stratas, and levels for increments within a strata

[Tuller]

One more thing to do

• [ ] I really need to implement bar defaults