Make panels collapsible

[selurvedu]

Expected Behavior

Being able to collapse and expand all panels at once with a single click or a hotkey, without the need to close them one by one.

Current Behavior

Currently, the desired behaviour is achievable only by closing each panel tab.

Possible Solution

Something like a Gtk.Revealer with GTK_REVEALER_TRANSITION_TYPE_SLIDE_RIGHT wrapping around NotebookPage, and toggling the state via double-click on a NotebokTab. However, I understand that this may not work because of the structure of PanelNotebook, so that revealer may need to be wrapped around PanelNotebook itself, and an additional UI element for toggling its state may be needed.

Environment

• Exaile Version: 4.1.2