Create RFC to Add opacity slider for color-picker

Please see issue #249

docs/RFC/0000-Add-Opacity-slider.md

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+# Add an opacity slider for colour-picker
+
+## To be Reviewed By:
+
+### Authors:
+
+### Status: ~~Draft | Discussion |~~ Active ~~| Dropped | Superseded~~
+
+Superseded by: N/A
+
+Related: Issue #249
+
+<br>
+
+## Problem
+
+What is this proposal solving? Why does the problem matter? How does the problem affect the user?
+
+## Anti-Goals
+
+What is outside the scope of what the proposal is trying to solve?
+
+## Solution
+
+Describe your solution and how it’s going to solve the problem. This is likely the largest section of your proposal and might even include some high-level diagrams if you are proposing code changes. While all important aspects need to be covered, also keep in mind that shorter documents are more likely to be read.
+Changes and Additions to Public Interfaces
+
+If you are proposing to add or modify public interfaces, those changes should be outlined here in detail.
+
+## Performance Impact
+
+Do you anticipate the proposed changes to impact performance in any way? Are there plans to measure and/or mitigate the impact?
+
+## Backwards Compatibility and Upgrade Path
+
+Will the regular rolling upgrade process work with these changes?
+
+How do the proposed changes impact backwards-compatibility? Are message or file formats changing?
+
+Is there a need for a deprecation process to provide an upgrade path to users who will need to adjust their applications?
+
+## Prior Art
+
+What would be the alternatives to the proposed solution? What would happen if we don’t solve the problem? Why should this proposal be preferred?
+
+## FAQ
+
+Answers to questions you’ve commonly been asked after requesting comments for this proposal.
+
+## Errata
+
+What are minor adjustments that had to be made to the proposal since it was approved?