Fix installation of private headers generated by wayland-scanner

Private qtwayland headers were not installed at first build, since
qmake was ignoring unexisting files from the install target. It
required another run of qmake to have a proper Makefile generated.

The rules for generated headers need CONFIG = no_check_exist, so that
files get listed in the Makefile even if they do not exist yet (thanks
to Loïc Yhuel for the pointer).

Change-Id: I1a0278d629295a55a3ddcf5f8fb068a04ba5be47
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Doc: Fix docs for QFontMetrics::height() to match code

After cb8445f0323b0eefbb04f1d8adad81a00b53abd8, Qt no longer
considers the baseline to have a vertical size, so it does
not add an extra pixel to the font height. The documentation
needs to be updated to reflect this.

Task-number: QTBUG-39668
Change-Id: I28fc813e21d73bb03f7055b0f0843511a12d308b
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>