applets/weather: Show forecast in two rows for envcan provider

The envcan provider (Environment Canada Service) produces a forecast model with 13 items (7-day forecast and 6 nights).

Show them on a two row layout, so its easier to visually parse and helps avoiding visual bugs on constrained spaces (system tray popup) due to a really long width.

Ideally, this would be better set-up by the dataengine but, since they will be deprecated soon and there are alreay too many indirection layers we can do an educated guess here.

Comparison between different providers:

In the system tray:

BEFORE	AFTER

BUG: 452792 BUG: 394809 FIXED-IN: 5.27

• First commit (not to be squashed): Simplify forecast layout: Use a simpler model and layout with just a delegate for every forecast (usually a day), instead of previous table-row driven approach.