Added header image

git-svn-id: http://plugins.svn.wordpress.org/black-studio-tinymce-widget/assets@579690 b8457f37-d9ea-0310-8a92-e5e31aec5664
Moved screenshots to assets folder
Added .git* files
Refactoring
Added ignored files
Refactor
Added deprecated functions for backward compatibility
Added deprecated function notices
Minor code improvements
Fixed compatibility issues with Page Builder plugin
Remove unnecessary script enqueue
Code formatting improvements after Scrutinizer check
Further code improvements after Scrutinizer check
Further code improvements after Scrutinizer check
Further code improvements after Scrutinizer check
Further code improvements after Scrutinizer check
Further code improvements after Scrutinizer check
Moved code for compatibility with other plugins on a separate class
Updated readme.txt
Moved code for compatibility with old WordPress versions on a separate class