[libcalamaresui] Reset font height when changing size

- defaultFontHeight() caches the result; clear cache
   when changing the default size, even though this happens
   only once in the current codebase.

src/libcalamaresui/utils/CalamaresUtilsGui.cpp

@@ -239,6 +239,7 @@ void
 setDefaultFontSize( int points )
 {
     s_defaultFontSize = points;
+    s_defaultFontHeight = 0;  // Recalculate on next call to defaultFontHeight()
 }