Let a crowded tab bar wrap into more rows instead of scrolling

[porky11]

A tab bar in a side panel is narrow, and everything stays on one scrolling row — eight tabs in a 220 px panel means four live behind a ▶. Towards #2, but by wrapping rather than a dropdown.

One opt-in hook:

fn max_tab_bar_rows(&self) -> usize { 1 }

The default is today's behaviour exactly, so nothing here is breaking. Above 1, a crowded bar wraps; if it needs more rows than allowed, it falls back to the single scrolling row rather than growing without end.

[porky11]

My application before the update:

After the update:

[emilk]

I really like this feature! But you should add some way to test it (manually), e.g. with a toggle in the advanced example.

[emilk]

this feels like a more safe pattern, no?

Suggested change

	}
	if output.rows == 0 {
	self.scrolling_tab_bar_ui(
	} else {
	self.scrolling_tab_bar_ui(

[porky11]

After adding it to the example, I encountered a weird bug where a red outline appears around the tab bar that appears for a single frame when you set the row count from a value above 1 back to 1 if only if one row is visible anyway.
I have to investigate it. Not sure when I get to it. Might be duplicate ID.