Draft: Cleanup powerstroke and fix Fill and Stroke use

Issues:

• Gradients cause crashes
• Deactivating powerstroke doesn't bring styles back
• Deleting powerstroke doesn't bring styles back

UX Changes:

• PowerStroke and PatternAlongPath apply stroke to their fills
• Fill is now optional, and it's deactivated by default because it's implementation is very unstable (needs to be re-written using groups)
• New preference to enable experimental fill option

Refactoring:

• New CSS callback in LPE, allowing mutating styles on setting
• Desktop style handles reading styles selectively for powerstroke and pap
• Desktop style isolated tool style generation and sp object setter.
• Lots of freehand tool code moved from C to C++
• Added opacity to fill-conversion and cleaned up creation code.
• Removed duplicate LPE style setting from pencil and pen, since powerstroke handles this internally.

Testing checklist:

• Pencil with tablet pressure enabled (powerstroke)
• Pencil with shape triangle in/out (powerstroke)
• Pencil with ellipse or clipboard (pattern along path)
• Pen with shape triangle in/out (powerstroke)
• Pen with ellipse or clipboard (pattern along path)
• Deactivate and delete powerstroke options
• Set stroke from color bar
• Set stroke from fill and stroke dialog
• Set stroke from paste style