DzlApplicationWindow should allow to reveal the header bar with a touchscreen
It is currently impossible to reveal the header bar on a touchscreen when
DzlApplicationWindow is in fullscreen mode: epiphany#814 (closed)
Either we allow to manually reveal the header bar so the apps can add the extra cases they want, or we make
DzlApplicationWindow handle that but it may be a bit too application specific.
If we offer accessors, we kill two birds with one stone as it would fix #38 (closed) too.