desktopGrid: Force allocation on reset()

When using gnome-shell 3.37 this was returning NaN,NaN:

let [proposedX, proposedY] = placeholder.get_transformed_position();

which then led to missing icons:

JS LOG: Error adding children to desktop: Not enough place at monitor 0

So now we force allocation of the grid including placeholders, to ensure that get_transformed_position returns usable coordinates.

Closes: #215 (closed)