Add "What's This?" to nearly everything in the main window

Summary:
This commit adds "What's This?" help to nearly everything in the
Dolphin main window (panels, views, buttons, ...). It adds the "?"
to the title bar so this help can easily be called.

For links in those help texts to work the WhatsThisClickedEvents are
handled in the main window class. This doesn't work for menus because
events from them aren't forwarded to the main window for some
reason so EventFilters are installed for the Control button menus.

Modifying the "Help" menu of KXmlGui is deprecated so no EventFilter
can be installed in the menubar. Therefore help texts without links
are provided for the menubar.

Test Plan:
Check if the event handling might make any problems.
Check for any big mistakes in the help messages.

Reviewers: Dolphin, elvisangelaccio

Subscribers: broulik, elvisangelaccio, yurchor, kfm-devel

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D20471