properties-window: Fix endless content size calculations

Deep count calculations are currently restarted with each "changed" event of NautilusFile object(s). This is not usually a problem if only one file is selected, but it is a pretty big issue when more files areselected. It is common that the calculation never ends. This is because the "changed" events are emitted in many irrelevant cases (e.g. free space change) and it totally doesn't make sense to restart the calculation in most of the cases. The initial idea was to react on ongoing file operations, however, the calculation currently doesn't react on file changes deeper in the tree anyway, or on changes, which happened after the calculation is done. Thus the current result can be outdated anyway. Let's ignore NautilusFile changes at all when calculating the content size as it is pretty impossible to implement this properly to dynamically react on all size changes in the tree.

Fixes: #363 (closed)