display warning/confirmation before activating on high poly meshes

[killarny]

Clicking to activate Modeling Cloth on high poly meshes can lock up Blender for a long time, which is especially frustrating if the button gets hit by accident (eg, I thought I had a low poly model selected, but it was a high poly one).

Displaying a confirmation dialog when activating on a mesh that is >20k faces (or some other good threshold) would be very handy.