[WebBrowser Plasmoid] Port to QtWebEngine

QtWebkit is old, unmaintained, possibly insecure, and also has quite a few bugs nowadays.
This also adds a couple of smaller features while at it:

• Reload button turns into a Stop button while loading
• Cursor changes shape over links
• Address bar selects all text when clicked to ease typing a completely new address
• Browser forward/back button on the mouse work

as well as a slew of features brought in by QtWebEngine, such as:

• Text selection support
• Context menu with "Copy link" and others

BUG: 371023
BUG: 361939
FIXED-IN: 5.13.0

Differential Revision: https://phabricator.kde.org/D10039