ApplicationWindow has the WindowFullscreenButtonHint flag by default

It should be possible to go fullscreen by default in most applications,
unless there is a reason not to, in which case the developer can turn
this flag back off.  It's also not so intuitive to turn it on, because
one needs to remember to specify Qt.Window as well, to avoid losing
the toolbar.  So making it the default avoids some mistakes/surprises.
Furthermore QWidget has this flag by default.

Task-number: QTBUG-33607
Change-Id: I350ffb38459d2a305353efe0db29335fe354a441
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
Reviewed-by: Liang Qi <liang.qi@digia.com>
Make sure menu bar has a parent window

On Android we depend on the parent window of the menu bar to
know when to show which menu bar. The menu bar parent was
set in the onVisibleChanged signal of the background item, but
this visible property is never changed. Adding a signal handler
for the visible property of the window as well fixes the issue.

Task-number: QTBUG-34681
Change-Id: I2cbafabe165420fd1697fbe51a8f393eb5158ffd
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>
TextEditor example: use the MessageDialog for the About box

Change-Id: I0813b7d392dfe7143d438b4057c5d787d223ad5e
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>