Add keyboard shortcuts for setting the background

[sbates-idrc]

Each background would have its own keyboard shortcut that can be used by an onscreen keyboard or other AT to set up an alternative interface.

Example shortcuts from the original Jira issue (using "w" for "world", the old name for backgrounds):

Clt+Alt+x,w,1 through Clt+Alt+x,w,9

Clt+Alt+x,w,a through Clt+Alt+x,w,z

Original Jira issue: https://fluidproject.atlassian.net/browse/C2LC-414