make non-API methods private for DocumentModel
extracted DocumentModel trait - the model itself now moved to EditDocumentModel;
initialised DocumentModel together with TheoryEditor state
use read-only document when the Theory is already loaded (exists in the heap): this way avoid errors when attempting to update loaded theories
new icons for Isabelle Theory editor (and a different icon when theory is already loaded)
if theory is loaded, make the editor read-only and change the icon to appropriate
refresh editor after initialisation
Merge branch 'loaded-thy-readonly'