Userland/TaskbarWindow: Show context menu on right click

[nyyakko]

**had to open the pr again because it seems like i messed up my previous fork? 😅😅

Just discovered and tried out the system today, and then I stumbled upon the fact that it didnt showed the classical context menu when i right clicked the taskbar. I found it quite unexpected, so i decided to implement one myself.

And thus this pr, which aims to add just that, came to be. It adds a simple context menu when the user clicks with the right button on the taskbar.

Right now its pretty simple and straight forward, but the idea can be easily expanded upon. Hope you guys like it :^)

[nyyakko]

I tried to merge my commits into one, as suggested by @gmta but i couldn't get it right. As you guys can probably tell, I am still learning so i'm sorry for the inconvenience :^(

[nyyakko]

The two commits can still be squashed, otherwise lgtm

done!

[bplaat]

Change the label because I want to see this get merged 😃