qt4: fix skins2 crash after exiting the preferences dialog
This fixes a regression brought by 7f485998 On skins2(tested on Linux), open and close the preferences dialog leads to a crash, because the whole qt4 thread terminates unexpectedly. This is due to setQuitOnLastWindowClosed set to true. As a dialog provider, qt4 doesn't run any windows, and therefore just closing the preferences dialog box (via accept() or reject()) signals the qt4 thread to quit.
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