initial import of backtrace browser plugin. not added to CMakeLists.txt yet.

What it does:
- you add e.g. the KDE/trunk checkout to the list of folders that are searched for source files
- the backtrace browser plugin will search for all files and keep an index of it
- if you open a backtrace (from gdb), it is displayed in a list view
- clicking on an item looks in the index and picks the (usually) correct file.
- this file then is opened in kate with the correct line

Backtrace navigation is pretty fast that way, especially if you have lots of code you maybe even don't know.