lockscreen: Add launcher plugin

I always forget to launch the music player when picking up the phone (and usually close it to save battery when not in use). This plugin allows to perform such actions without having to unlock the device via desktop files in a folder:

We currently just launch the app and hope for the best. A later iteration will have feedback, that won't make it into 0.33.0 though while this basic variant could.

To e.g. add a launcher for lollypop put a lollypop.desktop

```
[Desktop Entry]
Name=Lollypop
GenericName=Music Player
Comment=Launch Music Player
Icon=org.gnome.Lollypop
Exec=lollypop
Terminal=false
Type=Application
```

into ~/.config/phosh/plugins/launcher/.