testlib: Introduce MouseDoubleClickSequence() method

testlib: Introduce MouseDoubleClickSequence() method

The existing method MouseDoubleClick() emulates the mouse
double click event only.

The added method MouseDoubleClickSequence() emulates the full
sequence of mouse events a physical double-click would generate:
Press-Release-Press-DoubleClick-Release

Introducing a new method in order to provide convenience when a
test requires to simulate a complete double-click action without
changing the behavior of MouseDoubleClick() and risking to break
existing tests.

Add autotest.

Task-number: QTBUG-42185
Change-Id: I1cdddd9e21d3b1d8a818f6d4e3717b06b7d70e08
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>