Use user-space variables to pass temporary screenshot data

[fossfreedom]

Description

Screenshot uses a temporary folder path to save image data between the client and server dbus elements.

This PR ensures a user-space location is used rather than a system location.

Submitter Checklist

• [X] Squashed commits with git rebase -i (if needed)
• [X] Built budgie-desktop and verified that the patch worked (if needed)