[BUG #7180] removed explicit #use hints for themes
[BUG #7180] working version using an environment key for the list of poss. themes

- cannot use qx.Theme.getAll() in getThemes(), as the themes are (part-)
  loaded on demand
BUG #7180] switched part loading to qx.io.PartLoader

- instead of qx.Part
BUG #7180] simplyfied .getThemes() interface

- returns map instead of list of 1-element maps
[BUG #7180] simplify view.Header constructor to call selection.setSelection only once