add CLI options for better Dockerization

[bertsky]

In the Docker/broadwayd setting, it would be nice if we could arrange for the GUI to

• start maximized (like gnome-terminal --maximized)
• hide the Open button (so the Web access does not get to see the whole filesystem)
• hide the Edit functionalities (so the Web access becomes read-only)

[SvenMarcus]

We could also consider disabling the entire window decorations, see: https://valadoc.org/gtk+-3.0/Gtk.Window.set_decorated.html

[hnesk]

Together with commit 67b5316 this allows for command lines like:
browse-ocrd --fullscreen --restricted mets.xml or with short flags: browse-ocrd -fr mets.xml
There are four new options:

$ browse-ocrd --help
...
  -v, --version               Show version and exit
  -m, --maximize              Open in maximized window
  -f, --fullscreen            Open in fullscreen window
  -r, --restricted            Open restricted (no edit/open functionality)
...

I tried to disable window decorations as in #61 (comment) but the effect is quite similar to --fullscreen without filling the screen, and I think --fullscreen is the better option for any usecase, but I'm open to support disabling the window decorations, if you think it's still needed

Edit: renamed readonly to restricted, readonly sounds more edit-mode related than it is