This new invoker is able to handle all the palettes on a treeview,
removing the need of one invoker per cell renderer (CellRendererInvoker).
This simplifies the code and makes it more efficient.
Also removes the logic from the CellRendererIcon, which
should only care about drawing itself. [1]
Is important to note than in Gtk3 a CellRenderer is not a GtkWidget
then can't know when the mouse is over it or have the usual events
used by other invokers, making the implementation CellRendererInvoker
very complicate.
This commit also removes the invoker of CellRendererIcon.
The ScrollingDetector logic is simplified too,
because now there are only one invoker instead of one by renderer
and the example code updated.
[1] https://developer.gnome.org/gtk3/stable/GtkCellRenderer.html
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