Fixed properties collapse section to work with left/right panels.

Added undo/redo changes push for add section.

Added one panel demo link

Added one panel demo link and a list with features.

Added component tree for file manager.

Added download button.

Added file manager, new widgets components, collapsible panels for co…

…mponents and properties, new properties for html elements like padding, margin, typography etc.

Fixed unclosed div in editor.html and updated bootstrap 4

Merge branch 'master' of https://github.com/givanz/VvvebJs