Fix wxDataViewCtrl default row height in high DPI

Use FromDIP() to ensure that it is not too small when using DPI scaling.

Also make the code slightly more clear by using a meaningful variable
name instead of a (duplicated) comment.

src/generic/datavgen.cpp

@@ -2117,14 +2117,16 @@ wxDataViewMainWindow::~wxDataViewMainWindow()
 
 int wxDataViewMainWindow::GetDefaultRowHeight() const
 {
+    const int SMALL_ICON_HEIGHT = FromDIP(16);
+
 #ifdef __WXMSW__
     // We would like to use the same line height that Explorer uses. This is
     // different from standard ListView control since Vista.
     if ( wxGetWinVersion() >= wxWinVersion_Vista )
-        return wxMax(16, GetCharHeight()) + 6; // 16 = mini icon height
+        return wxMax(SMALL_ICON_HEIGHT, GetCharHeight()) + FromDIP(6);
     else
 #endif // __WXMSW__
-        return wxMax(16, GetCharHeight()) + 1; // 16 = mini icon height
+        return wxMax(SMALL_ICON_HEIGHT, GetCharHeight()) + FromDIP(1);
 }