Some optimizations and bugfix

New features:

• Support for .h header files.
• Target to 10.15 (now the standard swift library is not embedded).
• New interactive preview setting to allow the execution of js code inside the quicklook preview.

Bugfix:

• Removed markdown support in the quicklook extension.
• Fix little space for theme name on popover theme selector.
• Images of help files have been compressed.
• Added a margin aroud the preview on the quicklook window.
• Fast generation of theme icons.