[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.