Add configuration option to show Guake on the system taskbar

I would greatly appreciate having a **configuration option to enable the showing of the Guake window on the system taskbar** added, which, when enabled, should also cause it to appear on the Alt+Tab list.

# Motivation

I often need to switch between Guake and some other window back and forth repeatedly without Guake losing the focus so I can still see its contents. Currently, to switch back from Guake to the previously focused window without having to use the mouse, I have to press Alt+Tab, keep Alt pressed, and press Alt+Shift+Tab to move the focus where I need, which is super annoying.

It's the same issue as described in the issue https://github.com/Guake/guake/issues/2181. I just propose a different solution.

# Note

The Feature request issue template suggests using a website called FeatHub (https://feathub.com/Guake/guake) for reporting feature requests, but its contents look highly suspicious, so it should be probably reviewed and possibly removed.