Blender Git Loki

UI: FModifier layout updates, drag and drop

This patch implements the list panel system D7490 for FCurve modifiers.

The UI layouts are updated to make use of subpanels and to be consistent
with the rest of the interface, and easier to understand.

See the differential revision for screenshots.

This commit also significantly cleans up the FModifier UI code, and
improves, mainly by replacing the old button creation code is with
the newer interface API using RNA. In turn there is a bit of complexity
added because each FModifier has a separate panel.

Although reordering of FModifiers was not implemented before, we get
drag and drop basically for free here, so it is also included.
As noted in some older to do tasks, FModifiers aren't evaluated
in perfect order, which may be a point of improvement for the future.

Differential Revision: https://developer.blender.org/D7997