Themes & more

[texodus]

• Adds a few demo themes and theme selector support to the demo.
• Use fr instead of % for grid-template tracks (which makes for cleaner support for gap, but its still IMO a bit wonky due to how gaps are redistributed when a panel is undocked).
• Add padding and margin support.
• Redistribute panels proportionally on insertion, rather than 1 / N.
• Added a close part .
• Added top-level edge pinning (as seen in popular IDEs).
• Added resize cursor indicator when hovering over a split divider.
• Better HTML slot model does not need to generate named slots per child nor mutate the slot attribute on drag/drop, but you must now use the name attribute to identify panel children instead of slot.
• Various drive-by fixes, docs and tests.