Blender Git Loki

Outliner: "Purge All" function for Outliner in "Orphaned Datablocks" mode

Many users have been requesting a way to remove unused datablocks from the file/session
"without closing and reopening" Blender (or at least that's the impression I'm getting).

This commit adds a new operator (exposed as the "Purge All" button in the header of
the "Orphaned Datablocks" mode in the Outliner, which seems to be the logical
place for this) for doing so. It does so by wrapping up the save and "revert"
(i.e. reload the saved file from disk, without needing to quit Blender) operators
along with a confirmation prompt for good measure.

Caveats:
* Ultimately, we still cannot really cleanly delete any datablocks from the current
session outright without reloading the file/data at some point. Thus, we do need
to reload the file again before it can be used.
* This does mean that this operation is irreversible. Notably, Undo history is lost
is doing this operation. Hence the warnings... (Then again, undo/redo actually
reloads the entire scene DB from memory, so it's not anything uncommon ;)

Other Notes:
* The addition of this operator brings this mode more into line with being a kind of
"Trashcan" place, with this new operator being the manual "Empty Trash" button.
If the "Orphaned Datablocks" name is too obscure, maybe we could rename this
mode to "Trash" or something similar?