WelcomeService: turn the WELCOME.md variable into a constant

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
WelcomeService: Show a specific welcome message only once

We should avoid pestering users with the same message over and over again.
Better to show it once, remember the SHA-1 checksum of the displayed
message, and then only show the welcome message again when the application
version number *and* the welcome message changes.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add a note about the ticket to refactor SHA-1 code

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>