Add back the ability to customize the user agent

The new functionality doesn't use the old KCM to choose the user agent,
since it has been removed. A Konqueror-specific dialog to customize the
User Agent has been added instead.

The User Agent plugin has changed its function: since QtWebEngine
only allow to change the User Agent globally (we can't have different
User Agents for different pages) and statically (we can't change the
User Agent depending on the URL), now the plugin is used to
temporarily change the User Agent setting: when Konqueror restarts
again, the User Agent choosen from the configuration dialog will be
used.

Use QStringLiteral

Improve UI

Add identifications for other browsers


(cherry picked from commit 86e277f5)