As discussed in #128066, we want that activating a brush makes sure the
most appropriate tool is activated. For example activating an eraser
brush should activate an eraser tool, if available. This makes the
tool and brush binding two ways, which is more clear. Plus it means that
users don't have to care about tools at all if they don't want to, they
can just keep switching brushes and Blender takes care of the tool.
Implementation wise, this works by letting a Python operator lookup a
tool for a given brush type name, when activating a brush.
Design Task: https://projects.blender.org/blender/blender/issues/128066
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