Left mouse dragged + alt moves selected item

Another part of #36. Using alt instead of ctrl because ctrl indicates right drag which may get something to do later.
Added attributes to better signal intentions.
Fix meshesToSplit

This method returned nil if ownMeshSplitters was nil somewhere in the hierarchy. That was not intentional and caused issues when a scene expected a model to have more meshes (thanks to splitters) than it actually did.
Save a mesh's name in the model and make it editable.

Part of tackling #42.
Fixes #42.

The display name is now extracted properly, and set and stored within the file. To implement #41, it will be necessary to calculate the combined mesh name from the display name.