Sculpt: cloth brush constraint solver is now multi-threaded

* Used a simple method of assigning constraints
to threads.
* Constraints that couldn't be assigned are solved
in the main thread.

Sculpt: move more brush settings to brush channels

Fix brush defaults bug

Sculpt: Brush settings panel editor

* Moved brush settings (in sculpt mode) to
(for now) a new properties editor tab.
* Brush settings can now be individually configured
to show up in the workspace buttons.
* Brush settings can also be reordered.
* The new brush tab has a "preview" subpanel
to preview the workspace settings layout.
This is where settings are reordered.

Tweak more brush channel settings

Fix backwards preset in last commit

Sculpt: Added compile-type name checking for
brush channels.

Tune up a few hot loops revealed by profiling.

Just what version of C++ does the linux buildbot use, anyway.

Fix last commit

forgot a line

More linux fixes

Try to fix another linux compile error

try to fix weird gcc compile error

Sculpt: more brush stuff

Disable a static assert on gcc.

add missing files

Sculpt: Finish color support for brush channels

* BRUSH_CHANNEL_VEC3/4 are now implemented
* New flag BRUSH_CHANNEL_COLOR
* Also tried to sculpt's usage of wm.radial_control in keymaps

Sculpt: More brush engine stuff, got automasking to work with it

* Sculpt now has an API to get brush channel settings.
If a sculpt cache exists it will use the channels there
(ss->cache->channels_final), otherwise it pulls them
from a brush and Sculpt toolsettings. Exampes:

float f = SCULPT_get_float(ss, "setting", sd, brush);
itn i = SCULPT_get_int(ss, "setting", sd, brush);

* Improved the UI a bit

Fix small crash in last commit and a few typos