Update script so some of it can be sourced via .xprofile

[kurtlieber]

If we pre-populate environmental variables via .xprofile with the various backgrounds and desktops, that avoids file read/writes during actual desktop switching.

In theory, this makes the script faster. Need to code, test and validate assumption.

Will only be worth it if it substantially reduces the lag in switching desktops. This is the key part that needs testing.