Disable symlink/hardlink option when not relevant (Qt)

When the "Replace with links" option is not enabled, the choice of
symlink or hardlink is irrelevant and causes confusion. Implemented core
mechanism for controlling the enabled state of that option. Also
implemented the Qt interface for it. Cocoa-part is still to be done.

I used this opportunity to greatly enhance documentation of this part of
the code. I'm beginning to like documenting...

Ref #247.