Fill "Date modified" column on search tab

On the search tab of a repo it would be useful to see the last date when a file in some subfolder was changed. Currently the date column is empty for folders. Instead of this behaviour I suggest to recurse through the folders (or calculate it from the commits) and find the most current update date and use that one for the corresponding column.
This adds more overiew to the source tab and allows to navigate to fresh changes easily, even in deep directory structures.