Directory History list: show unavailable paths differently

[Slavic]

The directory history (arrow down button) may include extensive list of visited dirs, and some paths may be currently not available. For example, the paths from the previously attached USB drives which are not connected now: they start from the drive letter which is not assigned yet. I suggest to mark these paths in the list differently, for example, greyish instead of black (like the unavailable menu items). As a result, user will see at glance which paths in the history list are available right now and which aren't.

The check whether the drive letter is available (assigned) or not is quite easy. The check of availability of the full path may require more time and isn't necessary (but it may be implemented too as a configurable option).