How it works.

• Nothing changes by default (operators act as they did before)
• Operators can optionally set the OP_IS_MODAL_FOCUS_REGION flag. This means all modal() calls will use the region under the cursor (if it has the same region type as the initial region - to prevent strange problems), useful for quad-view.
• Operators can toggle the flag at runtime, because while dragging (for eg), you don't want the active region to change.

This resolves quadview issues for loopcut, knife and ruler.

Could be used to resolve issues for grease-pencil too. (didn't include in this patch but shouldn't be hard)