Change the word "application" to "app"

Historically, GNOME has called applications applications. However, the convention nowadays is to call them apps. We should follow this convention - it sounds more natural and will be more recognisable to users, plus it's shorter.

"App" and "apps" are the terms that are now used in the HIG, and it would be great if GNOME's interfaces did the same.

Examples in the shell:

• The "Show Applications" tooltip in the dash. This should read "Show Apps".
• In the power off dialog: "Some applications are busy...". This should read "Some apps are busy".

I'm sure that there are more!