Blender Git Loki

Added new button type: ROUNDBOX

This replaces drawing with uiRoundBox() or glRect() in button Panels,
which just fails for dynamic constructed button views where Panels are
sometimes invisible. Also eliminates hack with PanelPop().
Error was visible with new Modifier panel, entering/exiting EditMode.

Done Modifier and Constraints.

Syntax:

The last 4 free variables in the uiDefBut() call are:
- float, corner-rounding (0.0-pixels),
- not used,
- short: roundbox type (bits, 1 2 4 8 for clockwise corners starting left/top)
- short: color shade offset (color range 0-255)