This commit moves the list of transform orientations from scenes to workspaces.
Main reasons for this are:
* Transform orientations are UI data and should not be stored in the scene.
* Introducion of workspaces caused some (expected) glitches with transform orientations. Mainly when removing one.
* Improves code.
More technically speaking, this commit does:
* Move list of custom transform orientations from Scene to WorkSpace struct.
* Store active transform orientation index separate from View3D.twmode (twmode can only be set to preprocessor defined values now).
* Display custom transform orientation name in header when transforming in it (used to show "global" which isn't really correct).
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