mainWindow: Partly fix some animation glitches

These glitches are caused by the combination of HdyLeaflet and GtkRevealer
both animating at the same (and sometimes different) times. The revealer might
be part way through expanding before the leaflet decides it needs to fold, and
the same when unfolding.

Partly fixed by checking the widths of the leaflet's children, to predict
whether it will need to fold. This makes sure the headerbar animation is synced
with the leaflet's, but does not fix the other glitches.

A more robust fix would be to look into HdyFlap, but that has not yet been
merged into libhandy.
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