[GUI] Palette: Store a list of palettes

[rolandixor]

Description
Most apps in this field, including Inkscape, GIMP, and others, let you switch between preexisting palettes without having to load a file manually each time. Pinta can already let the user edit the palette, save, or load from disk, so there should naturally be a list of palettes that you can switch between while working, without having to do the manual loading step.

Additional context
The UI could be similar to what is found in Inkscape currently.

[rolandixor]

Related: #1989

[yioannides]

I think this is a good idea. One integral issue though is that Pinta still doesn't have a Preferences window for custom user paths, like project path, user color palette path etc.

@pedropaulosuzuki : assuming we had a Preferences window in Pinta for this sort of thing, we could reserve the Palette icon for a Inkscape-style popover with all available palettes in the user's palette directory, like so:

...and use a gear icon right next to it for #1989's tool menu:

EDIT: we've actually discussed this in #1362 and Cameron is not very keen on having a Preferences dialog, so this might be a non-starter.

[rolandixor]

I think a solution for this would be to have palettes be automatically loaded from a folder in the user's config or app data directory

[pedropaulosuzuki]

One integral issue though is that Pinta still doesn't have a Preferences window for custom user paths, like project path, user color palette path etc.

@pedropaulosuzuki : assuming we had a Preferences window in Pinta for this sort of thing, we could reserve the Palette icon for a Inkscape-style popover with all available palettes in the user's palette directory, like so:

EDIT: we've actually discussed this in #1362 and Cameron is not very keen on having a Preferences dialog, so this might be a non-starter.

As Pinta becomes more complex, it will eventually need a preferences menu. There are the language settings, for example, that should probably live there. Same for the menu bar option, that currently lives in the view menu and is a bit junky. Light/Dark mode should also be there, instead of View > Color Scheme, as well as themes (if we ever support them, same for icon themes). I don't think this applies for the palettes, tho. I'd rather not put it on the preferences window, but just on palette settings/configuration instead. Also, there's not much to configure in a palette except for variable names, colors, gradients and textures.

...and use a gear icon right next to it for #1989's tool menu:

I think the preferences menu could be a single entry in the hamburger menu, instead of a separate button. Of course, after we remove mostly everything from the hamburger menu and put it somewhere else, as suggested by #1428. Even so, @cameronwhite is probably waiting for more settings to be exposed and required before adding such menu.