Re-arrange the contents of the hamburger menu

To improve usability, the entries in the hamburger menu are
changed. Maybe the biggest fault of the previous menu contents were
that there were too many actions.

The new menu contents are composed of all the actions which are
necessary to use Dolphin and those which are very useful and should
be of interest for most users. Some menu contents change depending
on the state of the application. We can be more bold in only
showing what really seems necessary because this commit activates
the special sub-menu of KHamburgerMenu that helps users discover
all further features of Dolphin.

The hamburger menu is from now on also added to the context menus
in the view when both the menu bar and toolbar are hidden. This
allows users to hide both of them and still use all features of
Dolphin.