Introduce a history mechanism based on changes to Lepiter pages

[chisandrei]

At the moment changes to Lepiter pages are not tracked individually. This means that there is no undo at the level of a page for actions that change the structure of the page, like moving snippets. Also every individual text snippet has its own history that is not linked with the page. We should introduce a consolidated history at the level of a page that tracks all changes done to a page. This can be a basis for an undo mechanism for all types of changes done at the level of a page.

[seandenigris]

Cool idea! That would certainly be helpful

[chisandrei]

Wrongly assigned these commits to another issue:

https://github.com/feenkcom/lepiter/commit/faaa458c2268c959c8af1143fb74c6f8f51db193 https://github.com/feenkcom/lepiter/commit/81213ddb66723009edaea5abbca0e73736ad1837 https://github.com/feenkcom/lepiter/commit/33158fea2aeef6b0c6a8f74370430000dd58e1e3 https://github.com/feenkcom/lepiter/commit/b0e41e083cb104ca8b2b437f44ccf91043a9b3a0

[girba]

The undo mechanism is now enabled for text, Pharo code and examples snippets.