add to headerbar most frequently used source toolbar button to complete GTK3 implementation

[fossfreedom]

For headerbars the left hand side should contain the most frequently used source toolbar button

• https://developer.gnome.org/hig/stable/header-bar-menus.html.en

Now this varies between sources so we'll need to add a method to each source controller that identifies the correct source toolbar button and move it from the source toolbar to the headerbar.

This will need to be implemented after issue #71 - again an interesting mini project for someone if they are interested.

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