warnings in offscreen plugin, inherited from QPlatformWindow

Unnecessary-noise warnings can make tests fail, and aren't of much use to users either. So maybe these can be changed to qCDebugs or something.

For example QPlatformWindow::setKeyboardGrabEnabled() has qWarning("This plugin does not support grabbing the keyboard") and QQuickPopupWindowPrivate::setVisible() tries to grab the keyboard. (It even checks the return value to see if it succeeded or not!) So if a popup-related autotest is tested on the offscreen platform, this warning causes test failure for no good reason.

In that sense, warnings are almost as bad as exceptions.