Feedback from @gloook on new GUI

[BrunoLevy]

• labels and widgets are not aligned in tree
  • this will need ImGui table API that is not exposed through LUA, oh well... (need to work on Lua binding / generator)
• how to let the user drag a filename to a text widget ?
• picking vertices: locked vertices are not taken into account (makes a difference when locked vertices size is much
  bigger than standard vertices, may be confusing for the user) MeshGfx::draw_vertices_selection(), it's there !
  It's a bit annoying to do (this is why I did not do that in the first place). Maybe we can use filtering here as a way
  of rendering all vertices and keeping only the selection as rendered vertices. Is filtering supported on all profiles ?
  For vertices it is probably possible. For now, hw filtering not supported on GLUPES, and picking does not support array mode on GLUPES, but we can probably support both for GLUP_POINTS primitive
• crash on loading mesh with 0 vertex
• Cannot drag and drop files (.obj, .mesh) to graphite
• Same thing for texture image
  • fixed button for opening file selection dialog
• Point glyphs problems -> see Point glyphs s*ck a lot ! #58
  • completely new implementation that re-uses spheres shader
• history works well !
• transparent objects are not displayed when fullscreen-effect is active