Add level background

[alex-eg]

Closes issue(s): [#31]

Description of the changes:

Add support of scrollable modular background, file specification and loader, tests for all new functionality, and simple example in currently working code.

Code also revealed some issues with render priorities, like projectiles being rendered under the background despite it having the lowest background priority.

Reviewers: [@pkulev]

Task list:

• [x] submit pull request
• [x] review
• [x] testing
• [x] fixes after review, squashing, rebasing to master

[codecov-io]

Current coverage is 52.69% (diff: 97.78%)

Merging #36 into master will increase coverage by 9.31%

@@             master        #36   diff @@
==========================================
  Files            34         36     +2   
  Lines          1337       1615   +278   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits            580        851   +271   
- Misses          757        764     +7   
  Partials          0          0          

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