[libcalamaresui] Better default font size

- Previously, unless setDefaultFontSize() was called explicitly,
  the default size would be 0, leading to unexpected and weird
  displays (and a warning on stderr).
- If setDefaultFontSize() is not called, get a sensible size instead
  (like defaultFontHeight() was already trying to do).

src/libcalamaresui/utils/CalamaresUtilsGui.cpp

@@ -207,6 +207,10 @@ unmarginLayout( QLayout* layout )
 int
 defaultFontSize()
 {
+    if ( s_defaultFontSize <= 0 )
+    {
+        s_defaultFontSize = QFont().pointSize();
+    }
     return s_defaultFontSize;
 }