[WebBrowser Plasmoid] Support opening external links

This implements a context menu for opening links externally. There is unfortunately currently no way to add custom context menu
entries to WebEngineView's context menu, so the link menu is completely replaced.
It also lets websites open new windows and fixes target="_blank" links including a popup blocker.

CCBUG: 371023

Differential Revision: https://phabricator.kde.org/D11606