TEXTE_MODULE cleanup in preparation for text on different layers
- Removed the friends MODULE and FOOTPRINT_EDIT_FRAME from TEXTE_MODULE
  (as in the @todo comment)

- Refactored the Rotate/Flip/Mirror for text in modules into the
  TEXTE_MODULE class itself (members RotateWithModule, FlipWithModule,
  MirrorWithModule)

- New behaviour in coloring text on screen: reference and value still
  take the color from the MOD_TEXT_FR_VISIBLE and MOD_TEXT_BK_VISIBLE
  visibles; other text takes the color of the layer containing it
  (except when hidden, obviously) but still get its visibility
  controlled by those visibles (probably the most intuitive and useful
  behaviour)

- Still need to decide: should text on a layer be hidden when the layer
  is? Probably yes (still to be implemented);

- Some comment fixed and many cast converted to static_cast