Some improvements to the GP editor:
    - 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
Factorize Instance Vertex Attrib Setter
Factorize part of scenemanager
Add Instance type to some templates
Split material into single/dual tex
Fixed speed value at the kart selection start
Merge pull request #1530 from marcoll/master

Some improvements to the GP editor

Merged because it fixes some issues. Previously I wasn't able to create new GP without a crash.