Customize visible toolset

From: https://community.aseprite.org/t/suggestion-feature-request-allow-customisation-of-the-tool-selection/12064 (and probably other places)

We need a way to customize the default toolset defined in the <tools> element in the gui.xml file. It would be nice to have a Preferences > Toolset section to have a way to:

1. Customize the default toolset (group/ungroup tools, hide tools, etc.)
2. Customize other toolsets to be associated with UI layouts (probably for a future on-screen wheel or #1476 kind of customization, or #5855)

The first point is the most important, the second point is more related to other issues like #1476 or #4018 and might require more configuration (e.g. adding other buttons/actions, not only painting tools).

The configuration could go to the same user.aseprite-layout file, but the XML file should be re-structured a little (with backward compatibility). E.g. (and this is only an example):

<gui>
  <layouts>...</layouts>
  <toolsets>
    <toolset name="default">
      <group>
        <tool id="rectangular_marquee" />
        <tool id="lasso" />
      </group>
      <tool id="pencil" />
      <tool id="eraser" />
      <tool id="paint_bucket" />
    </toolset>
  </toolsets>
</gui>