glclient customization simplified

customization concept simplified.
- named theme concept removed.
- full glclient application is now customizable entirely
  the advanced adopters following the same old guide (and with full retrocompatibility)
  can now define an override file in the custom glclient for each app/ file;
  adopter will be advised to keep in sync changes that they see from the original file.
  this enables them to full override everithing solving various issues discussed with you all (@fpietrosanti @hellais @vecna)
- future direction will be to implement css override and text/translations override by        
  the admin panel of the web-app.