Blender Git Loki

Cleanup: Refactor `ed_screen_context()` to use hash lookups

Refactor `ed_screen_context()` to use `GHash` lookups instead of a
sequence of string comparisons. This should provide a nice speedup,
given that the hash for `member` only has to be computed once instead of
matching it to each possible string.

Reviwed by: brecht

Differential Revision: https://developer.blender.org/D9090