Add tooltips for the side buttons and main buttons

[novoselt]

"Main buttons" are Save/Time travel/AI assistant. When they are small (which happens always with split frames), it is hard to guess what they are doing. "Save" does look familiar, but works very different in CoCalc because of quite aggressive Autosave, so could use some clarification too. Perhaps even mentioning the current autosave interval and how to configure it.