Autosave when dirty

[bengaid]

• [x] create an experimental branch, from taipy_integration branch, allowing the xprjson to be autosaved when dirty flag is save.
• [x] add information save time
• [x] enhance date save format (use ISO)
• [x] add save version in meta

[GhilesHideur]

It's done. See issue_294 branch.

[bengaid]

• [x] add last saved date
• [x] remove save button
• [x] make everything variabilized (automatic save/vs manual save)

[bengaid]

Thanks Ghiles ! It is woking as specified in this issue. We shall handle live reload from another issue