Improve timeline clips color palette

This is a proposal to improve the timeline clips colors.

Here is a draft to build on:

Bonus:

• Make the audio waveforms clearer.
• Add ability to color tag clips.

See here for more info: https://phabricator.kde.org/T10085

https://community.kde.org/SoK/Ideas/2020#Project:_Promo.2FKdenlive