Remove a dependency on the old config module

This check might have have been needed in the past, but it is not needed now and can be safely removed.
Use constants for default thread pool numbers
Remove the old config module

The original Rana idea was that like in some window managers or in Emacs,
configuration files would just be valid Python code. This unfortunately
turned to be too fragile, so modRana got support fo classical ini-style/key-value
(but with nesting support) configuration files. Also more and more of the defaults
set at startup by the old config module moved to the constants core module.
With moving of the thread pool defaults to the constants core module the
old config module can be removed.
Make it possible to do automatic tile downloads with one thread only

This could be useful for resource constrained environments, for using tileservers
prone to banning parallel downloads and to potentially work around resource related
crashes like reported on the N900.

Also show a notification when changing the number of threads that to apply the setting
modRana needs to be restarted (it might be possible to remove this limitation in the future).