Some improvements to the GP editor

Second try, this one includes the suggestions made by konstin to the first one.
    - GP are now classified into groups, just like tracks: standard, user defined, and add-ons
    - allow empty GPs to be loaded by the GP manager, so that they can appear
      on the GP editor (but not on the track selection screen)
    - enable/disable buttons depending on the available options
    - provide some feedback after pushing the "save" button
    - some minor bug corrections and improvements