Export to cocos2dx and CSS, 3 new output image formats, refactoring

[sergey-shambir]

After this pull request, Qt5 and C++14 compiler required to build project (VS2013/2015, Clang or GCC5).

Changes:

• moved CLI interface code into separate file
• made refactoring to move metadata writer into separate class AtlasMetadataWriter
• added class CocosMetadataWriter to export into XML (Apple plist) format compatible with cocos2d-x
• added class CSSMetadataWriter to export into CSS format
• output image formats combobox now contains following formats (if supported by Qt image format plugins): PNG, JPG, BMP, DDS, WEBP, TGA, TIFF. On my Ubuntu machine only TGA format is not available.