Fixed: Invisible blocks when new-style texture pack is attempted.

External texture packs disabled for now.
Added scrollable menus to albow, and set them as a default for ChoiceButton s

The code was heavily inspired by the that of PaletteView. My defauls are not nessecarily the best.
Merge pull request #350
Submodule pymclevel updated.

Submodule pymclevel:
    > 66f7be2 - Debug: Move static def generating code into a function.
    > 66c9347 - Fixed: Incorrect SpawnPotentials structure when adjusting spawn positions during copy and import.