[Bf-python] EditMesh selection modes

[Tom M]

We should really have a way of setting mesh selection modes for this
release - it is simple to add, we just need an agreed interface and
where it should go.

If using flags to set multiple modes at once is out

perhaps we could do

Window.SetEditMeshMode('vert')

with the option of taking a list so we could do

Window.SetEditMeshMode(['vert', 'edge', 'face'])

LetterRip