[Lens] Dynamic Coloring Design Tweaks

[MichaelMarcialis]

This is a PR to address a handful of design-oriented issues discovered during the dynamic coloring feature review. Notable changes include:

• Changing "Color palette" label to "Color" to match designs.
• Corrected center vertical alignment for "Color" field contents.
• Changed "Value type" field to use a EuiIconTip and removed extraneous markup.
• Changed "Reverse color" from EuiButtonEmpty to EuiLink to better fit the space.
• Removed extraneous EuiFlexGroup wrapping the "Add color stop" button.
• Applied center vertical alignment to each color stop item.
• Applied a prop of compressed the "Color continuity"
• Changed nested EuiFormRow components that were wrapping each color stop item to be EuiFlexItem components.

Regarding the last bullet above, you'll notice two outstanding areas in color_stops.tsx marked with TODO comments. These reference commented out code pertaining to the invalid hex color error messaging. I'm thinking we should remove this commented out code for two reasons:

1. The individual color stops are no longer using an EuiFormRow component.
2. I don't think such error message are needed, as the EuiColorPicker dropdown already has the hex field displaying a similar error in the case of an invalid hex value.

Instead of having a redundant error message, can we instead change the current behavior so that if an invalid hex value remains on blur of the EuiColorPicker component, we simply revert to last known good hex value?

Otherwise, please feel free to merge if it looks good to you. Thanks!