Adds a bus signal watch to the default main context with the default priority (G_PRIORITY_DEFAULT). It is also possible to use a non-default main context set up using MainContext.pushThreadDefault (before one had to create a bus watch source and attach it to the desired main context 'manually').
After calling this statement, the bus will emit the "message" signal for each message posted on the bus.
This function may be called multiple times. To clean up, the caller is responsible for calling Bus.removeSignalWatch as many times as this function is called.