#55 automatically opened windows

[Oswald Buddenhagen]

the automatically opened editor windows should behave
differently for files inside and outside the project:
- inside the project:
- always open r-w
- if the file is already open, use that window
- outside the project:
- always open r-o
- automatically close them when the messages referring
to them are cleared
- assign window numbers above the directly focusable
range (20), if any at all