Add command to change debug level to the Debugger base class.

Currently, the debugflags can be listed and changed at runtime, but the debug level can not, which makes tracking down some bugs tricky, especially if they only manifest infrequently. I could have used this when trying to track down the BS2 British Museum Bug (http://sourceforge.net/p/scummvm/bugs/4828/)

Several engines have commands to do this, but I have been thinking about adding this for some time to the superclass of GUI::Debugger as it is a general problem.

This work was finally prompted by this being implemented yet again for the PRINCE engine in development for GSoC 2014:
https://github.com/lukaslw/scummvm/commit/8beb70bfe3af078feb21a43766808a25d9ae8dff

The following commits do this and remove the engine specific commands.

It should be noted that any out of tree engines should also be amended to remove any specific commands as these will also be redundant, but these changes should not break them in the interim.