Issue Summary

This issue is meant to track the feature request raised in #487 point 2 which deals with the necessity of having a customizable toolbar / widget to add different action buttons on demand.

This should allow the user to add any Pencil2D action that is exposed to the menus and / or to the shortcuts list to improve efficiency and flexibility in unique workflows and when using screen tablets.

This kind of feature is best used in conjunction with a full screen / canvas-only mode such as the one proposed in #1151

As reference please take a look at TVPaint Custom panels / toolbars. Here's a video on what they are and how to use them https://www.youtube.com/watch?v=qv1fpK9IItQ

If we reference an open source software think of Blender capability to create panels, though their process is quite more involved as it depends on an API to create the panels from scratch.

In Pencil2D however we could provide a simpler configuration interface to add the action buttons between columns such as the main menu editor found in the GNOME desktop