Writer: move inline nodes to toolbar inline btns

[distantnative]

Description

Summary of changes

• Include all nodes with inline: true not in the toolbar dropdown but as inline button (inserted between dropdown button and first marks button)
• Refactor k-writer-toolbar to get away more from node/mark naming but rather dropdown/inline
• Split a bunch of methods and added inline comments to make component easier to read and understand

Changelog

Fixes

• Custom writer nodes: fixed active state detection for nodes that include paragraph child nodes

Enhancements

• Writer: nodes with inline: true in their button definition are displayed as inline buttons in toolbar (instead inside the dropdown)
• New this.$helper.object.filter(object, predicate)

Ready?

• In-code documentation (wherever needed)
• Unit tests for fixed bug/feature
• Tests and CI checks all pass

For review team

• Add lab and/or sandbox examples (wherever helpful)
  • Writer: custom node with inline button sandbox#10
• Add changes & docs to release notes draft in Notion

[bastianallgeier]

Works great for me and the refactoring also looks good.