search wallpapers recursively in sub-folders

Submitted by Sergio

Assigned to Xfce Bug Triage

Link to original bug (#15671)

Description

Add a check-box besides the folder selector with 'Include sub-folders'.

For example, in Fedora we can install many backgrounds packages for current and previous OS releases and they set sub-folders of /usr/share/backgrounds like .../FXX/default or .../FXX/extras

Version: 4.13.4