lockscreen wallpaper support with command line option & assigning sources to lockscreen/desktop wallpaper

[KAGEYAM4]

Version of Variety you are using
0.8.13

Is your feature request related to a problem? Please describe.
I want to set lockscreen wallpaper completely independent from current desktop wallpaper set. i,e different sources and commmandline option.

Describe the solution you'd like

1. Wallpaper Sources can be assigned to 3 category, lockscreen/desktop/both
2. command line option, --[next|prev]desktop, --[next|prev]lockscreen, --[next/prev]both
3. set_wallpaper script split into two, so that user can modify it to there respective wallpaper backend.
4. Optionally, different duration for lockscreen & desktop or both using same duration.

Additional context
Similar #309 but deals with toggling current lockscreen wallpaper implementation where same background image is used for both.

As mentioned here #469 (comment), i could run 2 instances, but then there's two processes running, doubling the ram usage.