factored disabling out into own module
add the factored out module
generalized colours of the tower selection buttons a little bit
also generalized the resource level display colours
made the colours global. yes I know there are better ways
make the cursor colour depend on the selected tower (TODO later, we should show a dummy tower instead of just a rectangle)
enabled disabling/enabling of UI elements in waves/levels.
Merge branch 'feature/issue/#120_selectively_disable_functionalities_for_tutorials' into develop