[kcms/style] Improve GTK themes model code

• Renames m_themesList to m_themes, because it's a tree-based QMap, so it won't confuse developers.

• ItemModel::data() method:

  • uses simpler and sound checkIndex();
  • refactors from expensive QMap::{keys,values} (which construct a QList each time they called) to stl-style key/value iterator;

• Other accesses to themes now use const iterators too.