Allow to pick edges in DemoPicking

[sebmestrallet]

PR #102 follow-up, where I wrote:

Note: MeshGfx::set_picking_mode() also accepts MESH_EDGES, but I did not manage to read something other than -1.

I though edges were the facet edges (aka the wireframe), and the mesh I tested did not have edges. That is why I got -1 everywhere.

By allowing MESH_EDGES for MeshGfx::set_picking_mode() (the 9 loc of the commit) and by loading a mesh with edges (like CAD feature edges), we can pick edge indices with the demo executable.

Outside the scope of this PR, note that the mesh edges and the mesh wireframe (facet edges) are both toggled with show_mesh_ and have the same color mesh_color_ and the same width mesh_width_, making difficult to distinguish them: