[Bf-interface] More ideas on improving panel usability

[S A]

 Hi all,

recently there were a number of improvements for panels in
UI-graphical-redesign branch. I think we could go further and remove
drag grippers by joining expand/drag zones. I formed a mockup in
jquery (ignore the color of checkboxes):

https://jsfiddle.net/Lockal/uD5hu/8/embedded/result/

Interaction design explanation:
* When user generates a mousedown event on panel header, ui code
starts tracking the cursor
* If manhattan distance of cursor is above threshold (e. g. delta_x +
delta_y > 7) and LMB is still down, dragging starts at delta position
* If user release LMB below threshold, ui code treats this as
collapse/expand operation.

Note, there are also few important side improvements to be implemented:
* Highlight the panel header on mouseover (already done in
UI-graphical-redesign branch)
* Replace cursor to finger on mouseover to resemble finger drag paradigm.

The adaptation time is short! After finishing the mockup in jquery I'm
constantly trying to drag panels by header in blender, even though it
is not implemented yet.

--Sv. Lockal