Import 3D meshes from the GUI

[chapulina]

Gazebo-classic's model editor had an interface for users to choose a raw mesh file (COLLADA / OBJ / STL) and spawn a new link which uses that mesh as both visual and collision:

Some ideas on how to implement similar functionality on Ignition:

• From the component inspector, let users create a new visual / collision component from a 3D mesh.
• Create a new plugin which spawns a model with a single link that uses the geometry as both visual and collision.
• Make the geometry component editable on the component inspector, so users can choose a mesh as a new geometry.

[chapulina]

Drag and drop added in #939