[worksheet] properly update the coordinates of worksheet elements when moving them with the mouse

Description

When moving the element with the mouse (left button pressed), the move event doesn't have the information about the pressed button anymore (NoButton) that is needed in mouseMoveEvent() to decide if the element move was started or not. So, we check the pressed button in mousePressEvent().

Conformity

• Changelog entry
• Unit tests
• Update INSTALL
• Downport
• Fixes: #879 (closed)

When external dependencies are removed

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