RFR: 8294806: jpackaged-app ignores splash screen from jar file

[Alexey Semenyuk]

When `--main-jar` is specified and the name of the main class is picked from the jar's manifest, set `app.jarfile` property in launcher cfg file instead of `app.mainclass` and `app.classpath` properties to make app launcher run the app from the jar and not as a class from the classpath.

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Commit messages:
 - 8294806: jpackaged-app ignores splash screen from jar file

Changes: https://git.openjdk.org/jdk/pull/13413/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13413&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294806
  Stats: 96 lines in 4 files changed: 50 ins; 40 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/13413.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13413/head:pull/13413

PR: https://git.openjdk.org/jdk/pull/13413