populate-app.sh: we really, really do not have app/ anymore

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
populate-app: leave cached samples untouched

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

Migrating to WelcomeService in SciJavaCommon
Add WELCOME.md

Adds the file that will be displayed when IJ2 is first run.