This is issue 869 and is required for several reasons: 1) to make Inishell more
similar to MeteoIO, Snowpack, etc 2) because Qt6 is moving to CMake in
replacement of qmake 3) because it will "easily" allow creating native packages
for several platforms.

A wiki has been started for this.
Links to mio, sp, a3d, niviz are left in place at this point.

And minor cleanup

Now child windows' close entries are named better (close preview, close help)
and fit next to the main "quit inishell" menu.

Since we have a unified main menu, we do not want to add 'About' twice

And remove obsolete function

(but only from the help window)

...through keyboard shortcuts.

When opening context help the topic can now be anywhere on the page.
Also: QTabWidget::clear() clears the tabs but does not delete
anything... oops. Now we do this manually potentially saving big amounts
of memory and lookups.

Some further restructuring

This is an intermediate commit.

Add base functionality to the new Help window.
For now, it displays all content in one window.

Getting the tabs right will take a couple of commits... (an additional
abstraction layer is needed so that the tab bar can be exchanged and is
not coupled to the main window)