Incremental markdown parsing and various fixes

[hecrj]

This PR introduces a new Content type in the markdown module that allows users to incrementally parse Markdown—using its push_str method.

This is specially useful when you have potentially long Markdown streams—like an LLM reply in an AI chat application.

In fact, icebreaker has been the main driver of this feature (see hecrj/icebreaker#7):

2025-02-01.01-09-57.mp4

These changes also fix a bunch of minor issues and improve some APIs:

• The spacing of Markdown items can be configured through markdown::Settings.
• Big Markdown items have been split into multiple smaller ones to aid diffing and layouting.
• Functions asking for a container::Id or scrollable::Id now uses Into generics. These types also implement From<&'static str> now.
• text_editor no longer captures unfocus events.
• stack now propagates mouse movements unconditionally to all layers. This allows widgets that are being interacted with to still work even when the mouse hovers layers on top.
• rich_text now requests redraws properly when a link is hovered.
• Horizontal scrolling with mouse wheel now requires Shift pressed.

And I think that's about it!