Background Image Feature

[mkalygin]

• add support for background images of timelines in DB
• add support for background images on the client
• add UI for background images in authoring tool

[alterm4nn]

Sorry, this breaks copy&paste for timelines. need more testing.

[mkalygin]

@alterm4nn fixed the issue with Timeline DB schema. Now it should work.

[alterm4nn]

Merged manually as c582bdabc69d926bd130d57724a1d61b5c680ac2