Search for user specific config path and merge with the default config

Better guard against null values in user config
If the user set background cannot be loaded fall back to the default one
Merge pull request #143 from davidedmundson/custom_background

Restore custom background support from sddm-kcm
Fix xauth annoying error
Merge pull request #147 from Teknocrat/xauth-fix

Fixes xauth warning about authority file not existing.