Rename Tabs to Tab for clarity in the tab argument closures and in the method signature for tab_button

[hydra]

I found the fact that it has an 's' suffix on the name confusing when trying to work out the undocumented tab function while working from the tab_navigation.rs example.

Here's the livestream from today when I was working with this bit of code, see time offset: 1:49:31

https://www.youtube.com/watch?v=Q2VvQnRnHFQ&t=6580s

compare these examples:

let tabs: Vec<Tabs> = vec![Tab::General, Tab::Settings, Tab::Feedback]

vs

let tabs: Vec<Tab> = vec![Tab::General, Tab::Settings, Tab::Feedback]

and this screenshot, from the livestream, prior to making this commit.

and this change, from this PR:

https://github.com/lapce/floem/compare/main...hydra:floem:tab-navigation-example-naming-1?expand=1#diff-274a9e69291fc381bf841ad41b047acac861463d897f391b20c6ff42ce3bf02eL29-L30