On-demand scheduling with topbar button and keybinding

Implements #17 (closed)

First pass at "ondemand" scheduling. Please provide feedback.

Some notables:

• added a setting ondemand-source so the on demand option is kept when extension is restarted, user logs off/on
• shortcut/spin preferences are enabled/disabled inside of hidden shown mostly because I did not want to change too much.

Some question: The sources seem to be enabled twices: https://gitlab.com/rmnvgr/nightthemeswitcher-gnome-shell-extension/-/blob/master/src/modules/Timer.js#L116 and https://gitlab.com/rmnvgr/nightthemeswitcher-gnome-shell-extension/-/blob/master/src/modules/Timer.js#L58

This cause me to add a rather ugly workround: https://gitlab.com/goodwillcoding/nightthemeswitcher-gnome-shell-extension/-/blob/toggles/src/modules/OnDemand.js#L52-53

Is there a reason for double enable?