Debugger: Add toolchain combos to start dialogs, refactor detection.
Add combo box listing toolchains with debuggers to start external, attach and core. Another attempt at streamlining engine detection: Split in detection functions that first collect a list of available engines by preference, then remove disabled and wrongly configured engines and use the remaining best. matching. checkconfiguration is now the central place where engine detection and config check takes place. Rubber-stamped-by: hjk