Full path in systemd service

Use daemon full path in systemd service.

Issue: #8
Auto-set QPA plugin and desktop settings

Set QPA platform plugin and desktop settings automatically.

Issue: #11
Log greeter startup

Log when greeter starts.

Issue: #11
Enable Composite extention with Xephyr

In test mode we launch Xephyr instead of Xorg and we enable Composite
extension.

Issue: #11
Handle keys pressed on the password input field

MouseArea doesn't get key pressed events, but the password input field
does.
Fix status label alignment

Status label is now correctly aligned to the faces.
Remove leftovers

Cleanup code, no functional change.
Make everything disappear when users log in

Move contents inside wallpaper and turn root item to a black rectangle.
When a user is successfully logged in, set opacity to 0.0 with an
animation. Eveything will disappear leaving the black rectangle empty.
Don't use Qt Desktop Components for now

Keep using Fluid because for some reason we can't load standard icons
with Qt Desktop Components.