Add volume levelbar inline

[danirabbit]

Alternative design solution for #13. Instead of showing volume change confirmations as a notification, temporarily show an inline levelbar.

This is just a proof of concept. Todo:

• Support levelbars in the panel's css so we're not duplicating styles
• Don't show the levelbar when the panel first starts
• hide the levelbar when the indicator is open
• Don't show the levelbar when sound is changed via system settings
• Don't show volume change confirmations as a notification
• handle mic volume changes?
• volume blocking styles?
• insensitive style for mute?
• stay revealed when hovered?
• Test screen reader behavior