places-popover: add custom locations
This lets users specify arbitrary locations by latitude and longitude. There is some scruffy behavior due to some changes that will need to happen in libgweather's deserialization code but this is basically functional even without that.
Edit: The aforementioned scruffy behavior: after Weather is closed, opening it again will cause any saved custom locations to be displayed without a country. The fix is libgweather!257.
Edited by Ryan Hendrickson