Enhanced the cursor positioning/selecting text using the mouse in a textfield.

This also fixed an endless loop that could be entered by the previous code
in certain situations, after the patch to fix navigation/selection in a
textfiled with UTF8 chars was applied.

WINGs/Tests/wtest.c

@@ -572,6 +572,7 @@ testTextField(WMScreen *scr)
     field = WMCreateTextField(win);
     WMResizeWidget(field, 200, 20);
     WMMoveWidget(field, 20, 20);
+    WMSetTextFieldText(field, "the little \xc2\xa9 sign");
 
     field2 = WMCreateTextField(win);
     WMResizeWidget(field2, 200, 20);