Exclude some folders from sync

[wtextor]

Short version:
Allow deselection of whole folders in sync prepare dialog.

Long version:
In some cases I would like to exclude some folders from a sync operation. This happens when synchronizing source trees with embedded object folders. E.g. I have a source folder, where the makefile creates a debug and a release folder for the objects it generates. When synchronizing between a Mac OS X machine and a Solaris machine, these folders should be excluded, because the data will be of no use on the other site. I can see the folders in the sync prepare dialog, but I can only deselect individual files inside the folders, not the folders themselves, because the checkbox on the folder is disabled. Just enabling this checkbox and inheriting its setting recursively to its contents would be helpful. The skip regexp in the preferences does not help me here, because I would like to decide this on a sync by sync basis.

[dkocher]

In r3542.