Make frame display toggle button clearer in toolbar

[adamkewley]

Based on separate issues from 2 or 3 researchers.

Showing/hiding frames is dictated by an OpenSim::Model appearance property. If the user opens a model with them toggled off then no frames are shown.

OSC has a toggle button for this property---and related properties, like ones for toggling contact geometry etc.---in the toolbar but it is only an image icon, which users are unlikely to think about unless directly prompted (or it's written up in the tutorial, but that will only affect new users).

Therefore, it would be a good idea to make the toggle button a little more obvious. E.g. by adding text next to it like "toggle frames". This will be a little less tidy-looking than having just icons for each toggle, but toggling frames is much more important than the others, because users frequently (e.g.) mix up 3D UI manipulation gizmos with frames (they see the manipulator's arrows and think "that's a frame" rather than "that's something I can use to manipulate something - even non-frames").