Randomly assign background image for existing users and new users

[breezewish]

[twd2]

Why random?

[breezewish]

@twd2 For beauty: a monotonous background image (if using default values) is not nice.

[breezewish]

I suggest to preserve the script for future use so that when more backgrounds are added in the future we can randomize again ^_^TAT

[iceboy233]

Duplicate #246

[breezewish]

@iceb0y Not exactly the same. #246 wants to add a randomize button or a "random" option in preference page. This issue talks about the default setting for existing users and new users. It does not add a randomize button or a "random" option.