on_is_selectable and on_is_activable functions are now completely independent,
the former says if there shall be an icon in the left toolbar, the latter
says if the gizmo can be activated (by a shortcut or GLGizmoManager::open_gizmo)
It results in a six-fold reduction of the amount of data transferred into GPU because there is no need to duplicate vertices for every triangle. Also, normals aren't needed to be transferred to GPU.
This commit also fixed the following issues:
1) After loading a 3MF with painted triangles using the MMU painting gizmo, the painted triangles might not be displayed correctly in the MMU painting gizmo.
2) The MMU segmentation was unnecessarily executed for all layers and not just for the painted layers.
3) Object's base color wasn't changed when the assigned extruder for that object was changed while the MMU paint gizmo was opened.
4) Changing the base color of an object was only possible by removing all painted triangles.
