Fix a bug with pygments_style property

This property is a sort of duplicate of color_scheme, still here for historical and compatilibty reasons. If we changed the color_scheme without changing the pygments_style, the brushes are not upaded, we need to check that pygments_style is not different from color_scheme.name on every call to highlight block.
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there is a good reason for bypassing the runtime error (actually the runtime error is surprising at first but make sense when you know the document has already been rehighlighted).
Do not call clear editor when closing editor as this trigger the dirty flag

which is undesirable because this might trick collection widget in believin the new active widget is dirty while this is actually the old one that was dirty
Refactoring, documentation and bug fixing

Tested with more than 25 splits (and sub splits). It now works properly