applets/webbrowser: Implement pin button to keep plasmoid open

In my case, I have a little dashboard coded and want to view this in this plasmoid occasionally. Whenever I need to reenter the credentials, I find myself switching in between apps to copy the data from firefox/the password manager.

In those cases, it is very useful to keep the plasmoid open. Similarely, KRunner, Kickoff and the Systemtray also have a "keep open" functionality.