Remove "Import Meshes" from editor `File` menu

[adamkewley]

"Import meshes" should be removed from the osim editor's file menu once the "workflows" stuff is updated in the splash screen. This will also require double-checking the documentation, which may currently refer to this (to-be-deleted) menu.

The concept here is that the editor/simulator will, after #400 and other naming changes, become the main "workflows" of OpenSim Creator, but OpenSim creator will offer multiple workflows (e.g. mesh importer, mesh warping, frame definition, etc.). Users will be expected to open a new tab, which will automatically open the splash screen, which will list all alternative workflow tabs that they may open.

The presence of import meshes in the editor's file menu implies a stronger relationship to the editor than reality. The idea is to make it clear that importing meshes is a transient workflow/wizard, effectively.

note: although this is an easy issue, it depends on #400 being integrated.