Optimise intersections / List meshes

[eprana]

Work on #86 and #55 List meshes in interface and add a option to active a mesh in plugin.

Troubles with Maya callbacks on mesh creation : node is not completly created when callback is called.

[eprana]

Camera space positions in cache are computed for each current camera when needed (if not already in cache). But currently, these data are not removed when the current camera changed. I don't know if it is necessary since these information are cleaned when cache is rebuild, so quite often.

[eprana]

I still have some issues :

• Selection from Maya to UI does not works all the time. It go through the right functions, but item is not selected in UI
• Sometimes, items become white when unselected ...
• When we select items, we lose focus on listview and key shortcuts don't work anymore
• Scrollbar on mesh list disappear when reloaded

[eprana]

When we click on "Mesh" button, the list is reloaded. It happens because when a mesh is added, the list is not correctly updated since the callback is called while the node is not completly created. So we need to refresh the list.

[fabiencastan]

LGTM

[eprana]

@fcastan I don't know why setting color make the plugin crash, but it seems to solve the problem ...