file chooser: Store size more frequently

file chooser: Store size more frequently

We were only storing the dialog size on unmap, but resetting to the
stored default value more often, e.g. on focus-out. This was causing
the dialog to 'jump back' to its remembered size after the user
manually resized it, leading to frustration and bug reports.

Instead, save the dialog size on every ::size-allocate of the toplevel.
To avoid needlessly spamming dconf, only write the new value if it
changed.