stbt.py: Cleanup: Rename appsink element from "screenshot" to "appsink"
stbt.py: Cleanup: Rename appsink element from "screenshot" to "appsink"

"Screenshot" is the purpose of the element, but it is potentially
confusing because "screenshot" usually means a single frame once
captured. If you know GStreamer then you know what "appsink" is.

martynjarvis/stb-tester,stb-tester/stb-tester,wmanley/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,wmanley/stb-tester,wmanley/stb-tester