Set desktop background option when right click on image

[glpzzz]

It's not a personal requirement but two people have complained to me about this. I think it is subjective because depends on pcmanfm-qt handling the desktop (and it's not always the case), and if not, this will be unusable. I just left this here looking for opinions.

Expected Behavior

Right click on image and be able to set it as desktop background

Current Behavior

You need to right click on desktop in order to do this.

Possible Solution

Its already implemented with parameters for the pcmanfm-qt executable

[tsujan]

This can be done by making a custom action, using the --set-wallpaper option.

Closing because we don't add duplicate functionalities.

[tsujan]

For how to make a custom action, see the example in #40 (comment) and also #550 (comment)