Move GTK specific modes to GTK system

At the current time, these modes are not yet implemented in Electron. They may
be implemented in the future. Additionally, for many of them their behavior is
session wide instead of view specific. This would mean potentially proliferating
several views for each buffer. This is possible, but with debatable merit.

Additionally, they are removed from the manual/tutorial, which would otherwise
not compile.

renderer/electron: Properly calculate prompt buffer position

Consider the requested set height of the status/message buffer

mode/hint: Simplify calculation of colors.

help: Add button to `new` page for users to Install Nyxt.

foreign-interface: Automatically enable/disable page-modes in buffers.

base: Add emacs binding for next/previous tab.

Add variant of C-n and C-p with M-n, M-p for switching tabs.

source/*: buffers-set/get/delete -> buffer-set-get-delete.

Change the plural to the singular. The plural here does not make sense.

Merge commit 'refs/pullreqs/3626' into develop

buffer: Simplify buffer-source.

Delete buffer-initial-suggestions.

status: Use active-buffer of window, don't use current-buffer of brow…

…ser.