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.
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