refresh list of POIs (per garden) from server

[mfrasca]

in the 'about' window (which we have not yet programmed) you see a list of gardens, each presented with its name, the amount of downloaded panels, the amount of available panels, and a checkbox hinting the user at updating the downloaded panels.

in a way, each garden can be seen as a podcast channel. in this optics, where there are newer episodes, you may want to update. the interface would look something like this, where the refresh (⟳) checkbox is present only if updating makes sense.

garden name	panels	⟳
el cuchubo	0/21	☐
tanager	12/12
león ambiental	10/12	☐
parque centenario	9/9

anyway, in order to split this from issue #3, let's start with the 'about' window showing the list of gardens and the amount of panels (no total, no refresh).