Option to reset the colors

[Doublonmousse]

Add a button to reset the color palette to its default configuration.

Is your feature request related to a problem? Please describe.
Sometimes, whilst playing with different colors, we can end up with a color toolbar that is worse than the default on. For example, I changed it at some point and now I have two identical greens.

Describe the solution you'd like
A button (not sure where to put it though) to reset the color selection to its default state.

Describe alternatives you've considered

• Recreate the color palette manually
• have the windows that's used to change colors include the default color selection used in rnote so recreating the default choice of color is easier (not sure if this is the easiest way to do this, from the programming side)

[flxzt]

This would be something where I'd refer you to a graphical settings app to reset, but unfortunately for flatpak no equivalent for dconf exists so far..