Merge actual menubar widget into the Gnome-HUD.

[ripefig]

It should work like a menubar, but it should be activated on mouse-over OR on click, to improve usability. In order for the menubar to be activated on mouse-over, the cursor must move after the HUD is opened. This is to avoid accidental activation of the menubar by a cursor that happens to be positioned in the area where the HUD draws the menubar. This is the process:

Alternately, upon opening the HUD, the cursor can be made to jump to a specific location near the HUD popup but not directly over the menubar area. Below, the cursor is configured to jump just beyond the upper-left hand corner of the HUD:

The menus should always close when the user starts typing, to minimize interference with the search interface.

There should be a config option (in some file) to disable the activate on hover functionality and just use activate on click (like a regular menubar). If cursor jumping is used in the solution, there should be an option to disable that as well.

The solution should be implemented for both X11 and Wayland (Wayland solution will be compatible with Gnome Shell only).

[ripefig]

Any interest in working on this @jonian ? If so I can provide a much better design and we could try to raise some cash by advertising the issue on sites like reddit. People will pay for this. This could really really be a major step for apps on Gnome DE, both in terms of consistency and usability.

We need a place to put the clickable menu items to make this work, and the HUD is the only possible place for it. Plus this would be truly desktop and configuration agnostic (aside from the wayland branch) - no panels to deal with. Furthermore, this could be used to transform the menu into a composable shortcut interface, a la Emacs. If we get this right, we might even be able to augment the hud with arbitrary DBus calls and macros, so that it essentially becomes an extensible "Gnome overview" for apps. The potential is not to be underestimated.

There is simply no other way to make this work: server-side headerbars are a pipe dream given Gnome's attitude towards SSD and their top panel is hard-coded in place. App developers aren't going to ditch menubars or implement client-side HUDs. This server-side HUD is the only way to make things right.