When editing/showing a proposal or an investment, the most relevant
information regarding the marker are the coordinates. The title of the
proposal or investment is redundant because we already know the marker
is about that proposal/investment.
There's one problem with this approach, though: when editing a proposal
or an investment, the aria-label of the marker isn't updated
automatically when we move the marker to a different place. This
behaviour will only affect people who use both a screen reader and a
mouse, since keyboard users can't change the position of the marker in
the first place. We'll deal with this issue when we make it possible to
change the position of a marker using the keyboard.
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