Include global menu/preferences button in the window title bar

[waldyrious]

Is your feature request related to a problem? Please describe.
As a new user of Terminator, it took me a while after I installed it to figure out how to open the Preferences dialog (I had to open the right-click/context menu). This is not discoverable and makes the first experience harder than necessary.

Describe the solution you'd like

It would be nice if there was a visible "menu" button somewhere in the UI — for example, in the window's title bar. Here's how Terminator currently looks:

For example, here's Gedit:

Describe alternatives you've considered
Perhaps reusing the layout menu that already has a visible button in the titlebar (or a separate button next to it) could be an option, but since that titlebar is terminal-specific, rather than global to the entire app, I'm afraid that would make things confusing and muddy users' mental model of how the app works.

[mattrose]

I actually coded this a while back but dropped it due to lack of interest. There's a few ways to bring up the preferences window, I'm not sure we need another.