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