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Big UI refactor

• Header style revamped (thanks @aklevy)
• Much more things are shown on hover (headers, triggers, state creation little '+' (#433) etc)
• Many many small UI inconsistencies fixed - bad click areas, etc
• Intervals will now display a speed slider in their header when playing (#658)
• Intervals will now be grayed out when muted
• More vibrant green for execution color
• Colors are now set for the whole interval - slot headers and slot text are adjusted
• Events & timesyncs go through the whole height of intervals now which make execution clearer. (#568)