[minor] Comments + whitespace
Simplify weatherData().

This one needs more careful checking because I may have altered the UI.
I tried not to of course but I may have missed something.

Method weatherData was doing two things:
a) Populating weatherDataDico
b) Setting the tooltip.

I moved the tooltip related code to a separate method and removed redundant
attributes like "temp, tempFloat, meteo" etc.

I also renamed "Meteo" to "Clouds", although on second thought, "Sky" might
be a more suitable name.

Oh! And i removed the degree sing from the system tray. It was making the
letters too small on my system.
Convert "OverviewCity.unitsDico" to a class attribute.

Constants do not need to be calculated on runtime.