Enable output from the default logging category.

This way we can use qDebug for printf debugging again and don't
need to tamper with the logging filters.
Don't crash when getting a change signal without any active tool.

This happens esp. initially when there are no tools configured.
Use correct type, also fixes compiler warnings.

Might actually fix some bugs as well...
Re-introduce the org.kdevelop.kdevelop-$PID DBUS service.

This fixes some CLI integration scripts. We can now open files
in a running instance again, for example.
Make it possible to specify the session to open a file in.

Now, even with multiple sessions running, we can simply open a
file in one of these by specifying the session manually:

kdevelop -s session somefile