QIODevice: Improve read buffer use

Change buffer fill strategy to have more cached data for next read call.
This avoids unnecessarily many small reads from device implementation layer.

Change-Id: If1a039524afc03c02d2299babbfccef09f3f1cf0
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
QDesktopWidget: use qDeleteAll()

Change-Id: Icfbf47309e425deabcef81c8f11b8e42fcc9cdcc
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Simplify QWidgetPrivate::map{To,From}WS()

Change-Id: I065a173601d45ddb8ec52e721dfc596a487637fe
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
QWidget: simplify reading of an env-var in create()

Instead of initializing paintOnScreenEnv to -1 (thus forcing the
variable into the data segment), and then overwriting the -1 with
a read from the env-var, dynamically initialize the variable from
the env-var directly, thus allowing the variable back into the bss
segment (which doesn't occupy storage in the executable).

There may have been a reason to do it this way when the old code
could fail due to the memory allocation involved, but now with
qEnvironmentVariableIntValue(), that is no longer a reason.

Change-Id: I6547a81f69b0b57dc49bb4dd3ba1865ce8ac5b86
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>