Synchronize dirs: minor context menu issue

[astrotzky]

If the file exists only in the left directory, "view right" should be disabled.
If the file exists only in the right directory, "view left" should be disabled.

[ghisler(Author)]

Unfortunately it's not that easy to disable items in the menu, because multiple items can be selected. "View left/right" then opens all these files, you can switch through them with n/p keys.

[astrotzky]

This is not a big issue, but it would be pretty easy to solve it.
Just iterate through the selected items keeping two counters, one for the number of left files and one for the right files.
After that if the left files counter stays zero, the "view left" menu item can be disabled. The same goes for the right files.
Once again, this doesn't bother me that much though...