Support multiple independent source file buffers. Closes #14

 * Basic implementation:
    - All source files belong to an extra Gdl.Dock.
    - Currently there is no proper buffer_close signal handling (check if buffer
      is dirty -> dialog). Atm closing doesn't do anything.
    - "Interrupt" of closing a buffer is not properly implemented (tab
      position can change).
    - Not saved ("New document") view is not usable (completion and saving is
      not working)
 * Some minor improvements.
Merge branch 'multiple-buffers'

Conflicts:
	src/project.vala