Move source toolbar into the header-bar to complete this part of new GTK3 implementation

[fossfreedom]

At the moment the source toolbar is tucked away under a CTRL+T action.

For headerbars, the correct implementation should be to place a headerbar menu to the right hand side of the search button (https://developer.gnome.org/hig/stable/patterns.html.en) and when invoked a GTK Popover containing the contents of the source toolbar should be displayed as a horizontal button bar (https://developer.gnome.org/hig/stable/header-bar-menus.html.en)

Implementation considerations. At the moment that headerbar menu icon is used for desktop-environments to display the application menu where the DE doesnt support application menus.

Thus for these environments the headerbar menu button should display both the source toolbar button + the application menu ... is this even possible?

Anyway - an interesting self contained issue that could be an interesting mini project if anyone is interested in.

--- Want to back this issue? **[Post a bounty on it!](https://www.bountysource.com/issues/29596609-move-source-toolbar-into-the-header-bar-to-complete-this-part-of-new-gtk3-implementation?utm_campaign=plugin&utm_content=tracker%2F10046800&utm_medium=issues&utm_source=github)** We accept bounties via [Bountysource](https://www.bountysource.com/?utm_campaign=plugin&utm_content=tracker%2F10046800&utm_medium=issues&utm_source=github).