Mouse needs to be automatically grabbed when mouse button is pressed

QWidget::grabMouse() documentation states: "It is almost never necessary to grab the mouse when using Qt, as Qt grabs and releases it sensibly. In particular, Qt grabs the mouse when a mouse button is pressed and keeps it until the last button is released." Apparently this is default behavior in X11, but for Windows it needs to be explicitly implemented. Mac needs to be verified, may be necessary to implemente there, too.

Attached a test application to test various grab-related issues.

Original attempt at solving this was reverted as it broke linux platforms (https://codereview.qt-project.org/#change,34173).