Try to support all screen modes
fix mode 0 and implement mode 2
Added libz80-qt library

Added libz80-qt library based on libz80 from Gabriel Gambetta
(https://github.com/ggambetta/libz80).

Ported other code to new library. Now all Z80 opcodes should be
implemented.
Merge branch 'feature/big-refactoring' into develop
Merge branch 'feature/screen-modes' into develop
revert changes to main.cpp
clean up qimagescreenrenderer.cpp source
Merge branch 'feature/screen-modes' into develop
Fixed several warnings

Fixed warnings:

-> deprecated conversion from string constant to 'char*'
-> deleting object of polymorphic class type which has non-virtual destructor
Add GPL-2+ license headers
Merge branch 'develop'