PrintDialog: add a 'Close' button

The dialog was missing a 'Close' button so we add it.
Also change the mnemonic of the 'Preview' button, as it
was the same as the one for print 'Print'.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>

qt-ui/printdialog.h

@@ -25,6 +25,7 @@ private:
 private slots:
 	void previewClicked();
 	void printClicked();
+	void closeClicked();
 	void onPaintRequested(QPrinter *);
 };