applets/showdesktop: rename it "Peek at Desktop"

This applet sometimes causes user confusion because people coming from
platforms where the equivalent functionality minimizes all windows
expect it to do the same thing here. But it does not, and its name and
subtitle are sufficiently generic-sounding that they don't really
describe what it does instead.

Let's change the text to be clearer.

CCBUG: 456213

See also teams/usability/issue-board#12