Add true plugin system

[badcel]

• Use peas library (libpeas + libpeas-gtk)
• Put each plugin into seperate folder and create sepearte plugins (*.so files)
• Add compiler option to include each plugin seperately
• Add Window to dynamically show plugins (with option to enable/disable plugin). (libpeas-gtk?)

[badcel]

See: http://steef435.github.io/2016/01/20/getting-started-with-libpeas-extensions-in-vala/