GLMixer Code

Changed logic for deleting sources / selection.

Improved logic of selection / current source picking (Geometry and Mixing views)

Minor compilation fixes (AV_FORMAT deprecated stuff, forgotten comments, etc.)

Fine tuning selection and action mechanisms

Small change in selection behaviour; delete the selection without having to select one of its source, but also discard the selection when a new source is made current.

Implementation of selection operations using SelectionArea for GeometryView.
+ fixed management of non-modifiable sources
+ made selectionSource forward (selected first) by giving it max layer

Implementation of SelectionArea class to perform operations of mouse selection in Mixing and Geometry view (replacement of MixingView selectionRectangle).

Towards action keyboard shortcuts configuration ; created a method to list all actions of GLMixer

Implementation of visual overlay information for tools in Geometry View (center of rotation, proportional scaling diagonals, crop indication)
+ moved computeBoundingBox method (static) to GeometryView

typo