WallberryTheme: allow background change on socket notification

[delightedCrow]

Add functionality for the WallberryTheme background image to be able to change in response to socket notifications from other modules.

Discussed in #42

^{Originally posted by jdlucke November 5, 2021}
Would it be possible to configure WallberryTheme to be different picture selections for different pages? I am thinking that it would be interesting if I could place a sports-related image behind my sports modules pages, or a news theme behind the information pages...

[Andoramb]

Would it be possible to add a notification check function after this?
It could look like zis:

	notificationReceived(notification) {
		switch(notification){
			case "FETCH_PHOTO":
				this.fetchPhoto();  
				break;
		}
	},

Then just send FETCH_PHOTO:
http://192.168.1.XX:8080/remote?action=NOTIFICATION&notification=FETCH_PHOTO

[Andoramb]

Also...personally I think it's best to disable refresh picture on resume function, here
It requests new photo every time when it resumes.
This may be a good intention, but I want to keep the same picture when waking up the screen.
Just an idea :)
Now I see this was touched at Issue30