Merge branch 'blender2.8' into ui_layout_gridflow

Some more tweaks and cleanup.

Deduplicate some code, new solution to the fixed-estimated-size problem.

New GridFlow code was duplicating most of already existing func to
handle passing layout settings from parent to new child, fixed.

Also, added new 'variable' flag to uiLayout, set for GridFlow and all
its children, that enforce items to always estimate their real ideal
size, and not some arbitrary large fixed value.

Merge branch 'blender2.8' into ui_layout_gridflow

Some cleanup/refactor from code review.

Merge branch 'blender2.8' into ui_layout_gridflow

Conflicts:
release/scripts/startup/bl_ui/properties_texture.py
source/blenderplayer/bad_level_call_stubs/stubs.c

Merge branch 'master' into ui_layout_gridflow

Initial (and theorical) code for grid flow layout.

Needs RNA API, and... testing! :P

Painfully enhanced columns/rows computation to make best use of available space.

Now layout will use less columns than possible, if it does not increase
number of rows. Allows to fill more evenly available space.

Also fixed some quirks and corner-cases issues.

Add gridflow layout RNA API

Fix gridflow layout logic, and building with bplayer.

Also add the 'modulo' option for columns number (i.e. set number
automatically, but only allow multiples of given number).

Notes: changes to properties_data_mesh.py are just for testing
obviously, will be reverted for final patch!

Fix logical error, in our current UI height is not really adaptative, so we want max to work with heights, not average ones.

Also, add another experimental example of gridflow layout usage (needed
some hacks/tweaks to low-level UI code, by default our buttons tend to
get way to much width than actually needed!).

Factorize gridflow computation code.