Add better support for Palettes

[nicogig]

Currently, the colour picker always modifies the first colour in the palette. Moreover, there is no actual indicator in the app for which colour in the palette is being modified, nor there is a view of the colours in the palette. These features should be added in Milestone 2.

The MVP should be the ability to view the colours in the palette, be able to select a colour, and be able to modify it accordingly.

[nicogig]

The MVP mentioned here was a reference to Primary/Secondary Colours, which has been satisfied per the last commit. Palettes cannot be altered, or so it seems.