applets/weather: Make desktop widget big enough to fit Configure button

If we don't set Layout.minimum{width,height} properties on the
FullRepresentation, it will default to something tiny and the
Configure… button shown on first launch will overflow.

Set it to a sane size, and also take into account the final layout once
content later gets added so it isn't too small once configured.

BUG: 419591
FIXED-IN: 5.24.7
2 jobs for !216 with make-desktop-widget-big-enough-to-hold-the-configure-button in 1 minute and 31 seconds (queued for 3 seconds)
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