Blender Git Loki

Display enum descriptions in tooltips for operators using a "type" or "mode"
property

Changes:
This commit adds a second line to the tooltips (below the generic operator
description) showing the appropriate description for each enum option. This
brings it more into line enum properties in Blender which also show this sort of
information.

Rationale:
Operators such as Snap and Mirror in the Action and Graph Editors use an enum to
control their behaviour (respectively, "how to snap" or "what to use as the
mirror line"). In the menus, these options are displayed using a submenu, but
hovering over each of these items for more information from a tooltip only shows
the (relatively unhelpful) generic operator tooltip/description.

Another area where these descriptions are useful is for Keying Sets, where it's
now possible to see the descriptions for what each Keying Set
does/affects/requires. Again, this is more helpful than just the generic
tooltip, which would be something like "Insert keyframes using a Keying Set".