Add a setting to create custom quick color buttons

[colebot17]

This feature allows custom quick colors to be added from the User Interface settings page.

A string setting holds comma-separated hex values in local storage. When the UI is updated, each color is added as a button underneath the existing colors. With lots of colors, the number of colors per row matches the existing alternating row pattern.

Testing:

• Invalid hex codes are not added as buttons
• Extra spaces around the commas have no effect

Limitations:

• The user must edit the string to change the colors
• There is no color picker UI available

[DedeHai]

Thank you for the contribution, this is a nice feature, however I do not see it fit for merging as a standard feature:

• colors are stored locally, meaning only available on the device you created them, correct?
• not using the color picker is not user friendly