Make dialogs shortcuts be coherent

[orontee]

Context

See #154

Current shortcuts aren't coherent between preferences, play random tracks, play stream, about, etc. dialog. They're not coherent with other Gnome applications.

Analysis

Goal (motivated by "coherence" with other Gnome applications):

• Have Escape behave the same with all dialogs: It closes the dialog
• Have Ctrl+w close the last application window (and there's only one, thus same as Ctrl+q)

Other goal (unfortunately abandonned, see below):

• Have keybindings be reconfigurable

Reconfigurable keybindings

Reconfigurable keybindings in Gtk must be implemented using "keybinding signals".

For example there are "keybinding signals" to close dialogs and shortcuts window; Their default binding is the Escape key, see Dialog::close and ShortcutsWindow::close

But I found no way to get in Python the Gtk.BindingSet instance required as first argument of Gtk.binding_entry_add_signall() (BindingSet.by_class() isn't exposed to Python through gi according to its documentation, see https://docs.gtk.org/gtk3/type_func.BindingSet.by_class.html#description.

Thus:

• I had to use an accelerator or to connect to the key pressed signal for the preferences window (the second option has been selected, arbitrary decision); In both case the binding can't be reconfigured at runtime
• Same remark with the Ctrl+w implementation from #154, the binding can't be reconfigured at runtime
• As a consequence, the behavior expected by @hufman can't be implemented by "reconfiguration" with current implementation...

[orontee]

Something like the following is possible to add a Ctrl+b key binding to close dialogs (play stream, play random tracks, etc.):

@binding-set CloseWindowFromDialogs {
  bind "<Primary>b" { "close" () };
}

dialog {
  -gtk-key-bindings: CloseWindowFromDialogs;
}

window#preferences-window {
  -gtk-key-bindings: CloseWindowFromDialogs;
}

when a "keybinding signal" named close has been created on the class PreferencesWindow:

    @GObject.Signal(
        flags=GObject.SignalFlags.RUN_LAST | GObject.SignalFlags.ACTION,
        arg_types=[],
        return_type=None,
    )
    def close(self, *args) -> None:
        super().close()

:warning: Not working with w char due to its use as accelerator in app.py... See https://lazka.github.io/pgi-docs/#Gtk-3.0/classes/Widget.html#Gtk.Widget.add_accelerator where it's said that such accelerators aren't editable at runtime.

My understanding is that BindingSet.by_class() is not exposed to Python, thus there's no way to define key bindings that are editable through CSS... See https://docs.gtk.org/gtk3/type_func.BindingSet.by_class.html#description