Mouse cursors now work again
- centralized screen-level cursor changes, no more operator
running for it.
- spacetypes have callback to check/set individual cursor
types. Use notifier SCREEN_CHANGED to make sure it works
on mode changes etc.
- new calls WM_cursor_modal() and WM_cursor_restore() to
make temporarily cursor types during modes.
- used above for view2d cursors.
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