Set mesh or texture to nothing

[i-make-robots]

Work on #225

New mesh selection process

1. Add a MeshInstance.
2. Click the MeshInstance.
3. Click the ... selection button
4. Click Load Mesh and locate a mesh in your file system. Mesh Chooser Dialog is same as before.
5. Click to select your newly loaded mesh. Note all loaded meshes will appear in this list.
6. Click Clear to set the selection to nothing.
7. Click OK to confirm your selection.
8. If you have selected a mesh, it will appear at the world origin.

• If you have moved the MeshInstance it may be somewhere else.
• If you have 3D View > Render > Meshes turned off it may not be visible.

[i-make-robots]

Added texture chooser dialog. Same idea.