projectexplorer: Make the debugger language chooser a (hardcoded) "aspect"

This is the first step of a larger refactoring. The plan is to make
debuggersettings more orthogonal to individual runconfiguration
implementation. This patch alone already pushes the settings handling
to the debugger plugin and removes code duplication in the
runconfiguration implementation.

Change-Id: I4c78d1658ea462d3df14b873f8f41cc918a23f1a
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>
Reviewed-by: hjk <qthjk@ovi.com>