QScroller with QScroller::TouchGesture does not cause widgets to ignore synthesized mouse events

When using `QScroller::grabGesture(widget, QScroller::TouchGesture)`, the widget captures the touch gestures but the system synthesized mouse events aren't ignored by the widget. This can cause some weird behaviours for certain widgets.

Using `QScroller::grabGesture(widget, QScroller::LeftMouseButtonGesture)` produces the expected result.

To reproduce:

1. Use Pixelator example https://doc.qt.io/qt-5/qtwidgets-itemviews-pixelator-example.html as base
2. In `mainwindow.cpp`, add `#include <QScroller>` to the top, and the following at the end of `MainWindow::MainWindow()`:

view->setVerticalScrollMode(QAbstractItemView::ScrollPerPixel);
view->setHorizontalScrollMode(QAbstractItemView::ScrollPerPixel);
QScroller::grabGesture(view, QScroller::TouchGesture);

1. Build and run program
2. Use touchscreen to scroll the `QTableView` by dragging inside it, notice that some of the cells gets selected, also there may be some jumping due to bugged autoscroll
3. Change `QScroller::TouchGesture` to `QScroller::LeftMouseButtonGesture`, rebuild and run program
4. Use touchscreen to scroll the `QTableView` by dragging inside it, notice that the cells doesn't get selected. Same with mouse dragging.