PR: Add custom palette creation api

[goanpeca]

Fixes #151

We have now an api to create a custom theme programmatically :-)

This will:

• Create a folder with the name of the style
• Copy the QSS folder
• Copy the RC folder
• Create the _variables file
• Create the palette images
• Compile the SASS code
• Return the stylesheet

See https://github.com/ColinDuquesnoy/QDarkStyleSheet/pull/161/files#diff-4258867fbbf6efefd2605056768f8dfbR126

The result of running the method is:

@dpizetta

[dpizetta]

Awesome! Tks @goanpeca