Add QAbstractButton::textAlignment property

Currently, QPushButton text is always centered, unless it has menus attached, in which case it becomes left-aligned on macOS.

Some applications might want to change the default alignment.

According to vhilshei (see QTBUG-107835): "In macOS's AppKit, all buttons have an alignment property, which defaults to left for menu buttons, and to center for normal push buttons. Then applications can explicitly set the property depending on the context in which the different buttons are used."