Handle button-press instead of button-release

It seems to be more common for tray icons to handle user actions in
button-press-event, not in button-release-event handlers. This commit
merely adopts this behavior.

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Fix declaration of `status_icon_on_button_press'

A callback function must return a boolean value to indicate whether
propagation of an event should continue after the callback returns. The
button-press-event handler was previously declared to return void which
means the behavior was more or less undefined.

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Fix button event handling when scale is visible

When volumeicon was configured to show the scale window on left-click
events, clicking the icon again once the scale was visible would mute
the output. This commit makes sure the scale window is hidden again in
this situation.

Signed-off-by: Niklas Koep <niklas.koep@gmail.com>
Merge pull request #29 from nkoep/icon-button-event-fix

Rework button event handling