keybindings should extend beyond what gtk CSS allows
@chergert
Submitted by Christian Hergert Link to original bug (#749331)
Description
Today, keybindings are supported via gtk-key-bindings CSS extension.
This does not cover all of the use cases we need. It also needs to be overly verbose in that we need to map the accelerator for every widget. And sometimes, the widget we want to map to doesn't have a "action" helper G_SIGNAL_ACTION to help perform the task.
Therefore, we should extend GbKeybindings to also map a GtkAccepMap, also based on mode.
We can then add a mode for emacs, vim, default, and possibly others.