Merge branch 'blender2.8' into temp-tab_drag_drop

Cleanup: Refactor button-group code

* Split generic button-group code from specific application
* Move button-group API to interface.c.
* Avoid struct abuse by using wrapper structs
* Naming, comments, etc.

Fix error in reordering after new items were added.

Fix error in last merge

Was freeing memory too early, it will be correctly freed a few lines later.

Merge branch 'blender2.8' into temp-tab_drag_drop

Merge branch 'topbar' into temp-tab_drag_drop

Merge branch 'blender2.8' into topbar

Merge branch 'topbar' into temp-tab_drag_drop

Merge branch 'blender2.8' into topbar

Keeping the branch for now, I might delete it soon though.

Fix unintentional changes from merge conflicts

Merge branch 'blender2.8' into topbar

Move data of global areas into a separate struct

Made the struct accessible via a pointer in the ScrArea so that we can
use a simple NULL-check for this pointer to see if the area is global.

The struct is not written to files, meaning we have the freedom to do
changes to it without having to worry about compatibility.

Support hiding the lower sub-bar by dragging the area edge up

Fix hint for makesdna to not write top-bar struct commented out

Just did this for testing, but should not have ended up in the commit.

Bring back unintentionally removed error print

Topbar: disable redo properties in topbar entirely for now, behind #ifdef.

This is going to be different with the tool / command separation plans, so
let's not confuse users for now.

Fix error from one of the merges, this line should have been removed.

Tweaks to topbar versioning code

* Don't write global area pointers at all, leaves .blends a little
cleaner without broken pointers.
* Don't call editors functions from blenloader.
* Fix black topbar when loading some existing 2.8 saved .blends.

Merge branch 'blender2.8' into topbar

Fix typo in #define