Emit seatCreated() signal when listing seats

When the seat manager starts we list all the seats available and now we
emit a seatCreated() signal which will be caught by the LoginManager
class.

Issue: #8
Add logPath property

This way we know the path where we are going to save more log files.

Issue: #8
Start system compositor process

Run and detach system compositor process in order to run the greeter.

Issue: #8
Print fatal messages to standard error

These are so important that we want to always see them.

Issue: #8
Simple display class

Simple display class to run the greeter.