Change "default notebook" into "remember session"

[Jaap Karssenberg]

Instead of having a default notebook opening when zim is started have an option to open whatever notebooks were open in the previous session. In case you have a single notebook which is the default htis will always open. In case you usually have multiple notebooks open, they all open on next start showing the full state of whatever you were doing.

* Needs support from the daemon to keep track of state
* Needs way to really close a notebook, even in daemon state, also see bug #291339

[Jaap Karssenberg (jaap.karssenberg)]

Will redesign "open notebook" dialog, but not going for global state after all. Doesn't feel right when testing. So flagging this idea as "won't fix".