Collective UI/UX rework/standardization tracking issue

Background

There are a number of open UI/UX issues that will require breaking changes, and should therefore happen before BuildStream 2.0.

This issue is here to track progress on them, and ensure that we keep them in sight.

Task description

The following issues should be resolved as part of a UI/UX rework:

• #345
• #685
• #771
• #773 (closed)
• #819
• #1065
• #1048 (closed)
• #1067 (closed)
• #422
• #822 (closed)
• As part of this rework, we should also set our design principles for these things in stone - this should at least be documented in CONTRIBUTING.rst.
• #1349

Acceptance Criteria

BuildStream's UI/UX is reworked and (hopefully ;)) shouldn't require future breaking changes.

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