NavigationWidget: Add menu to split button

This is more consistent with the split button that we have for the
editors, and the default split behavior, which duplicated the current
view, seldomly resulted in what was wanted anyhow.

Change-Id: Iaba62151edbf11f6bbfaf45d68371532bff48dd5
Reviewed-by: Daniel Teske <daniel.teske@theqtcompany.com>
Help: Clean up automatically registered documentation

It would keep documentation registered as long as the files are still
there, but that is actually not what is wanted. If you remove a Qt
version from Qt Creator, or switch between different Qt Creator
installations with different auto-detected Qt versions, the registered
documentation should be updated.
This also reduces the number of open files in these cases.

We keep a list of manually added documentation in the settings and treat
all other documentation as automatically managed.

Change-Id: I542f9aee0e62cab70db5afc5c6d6764d84cb253d
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>