Have the GLWidget manage the scene/molecule

Moving convenient API from the Avogadro MainWindow class to the
GLWidget.

Change-Id: I98b8c14f62fd02fcf6d4eb15241181ae3cb22929
Have the model take ownership of the ScenePlugin

Change-Id: Icb1a1c24fd0f78ae3a9c5587a0f64d714855a2d7
Documentation clean up

Change-Id: I4dcd4c1a1e0b8bc3431ce6c29262721bd321844d
Make update/clear scene slots, manage connections

Made the updateScene and clearScene methods slots, set up the
appropriate connections internally so that more of this can be removed
from the application logic.

Change-Id: I5f32daa78cd2cdce13ac095bb49000e9b87c8bb9
Merge topic 'glwidget-scene' into master

e4e21164 Make update/clear scene slots, manage connections
40c30129 Documentation clean up
581afeeb Have the model take ownership of the ScenePlugin
08195e27 Have the GLWidget manage the scene/molecule