applet/webbrowser: add persistent storage
This adds persistent storage to the webbrowser applet.
Sometimes you want to be able to store a state of an website like e.g. login data.
This creates a webengine browser profile stored in ~/.local/share/plasmashell/QtWebEngine/org.kde.plasma.webbrowser/ and introduces the option to turn private browsing on and off (defaults to on like the behaviour in the default profile).