Allow tab to have no active tabs.

[hydra]

This is a common scenario when using a tabbed interface with one-tab per document without any documents having yet been loaded.

A running example using this can be found here:

https://github.com/hydra/floem/tree/tabbed-document-ui

I'm new to Floem, likely I made mistakes, so review this PR carefully.

[dzhou121]

Do you mean for cases that tab children is empty?

[hydra]

Do you mean for cases that tab children is empty?

yes, I note that in the tab implementation, there is a default value of 0 for the index for the active tab, as opposed to None. See this change:

https://github.com/lapce/floem/pull/592/files#diff-7baac154f944b70ba151fa8121dc250311a7d472e5f14a485c48a02e749a6dd3L88

It's possible I'm missing something, as the API to the tab() isn't documented and it was very unclear to me what the purpose of 3rd argument (key_fn) was supposed to do, and wasn't obvious when looking at the layout example which is the only example that calls the tab method and always has a non-zero amount of tabs.