RFR: 8231591: [TESTBUG] Create additional two phase jpackage tests [v4]

[Alexander Matveev]

> https://bugs.openjdk.java.net/browse/JDK-8231591
> 
> - Added MultiLauncherTwoPhaseTest which uses predefine app image with multiple launcher and tests to make sure installer
>   will create shortcuts for all launchers.
> - Fixed Linux DesktopIntegration to create shortcuts for additional launcher if we using pre-define app image.

Alexander Matveev has updated the pull request incrementally with one additional commit since the last revision:

  8231591: [TESTBUG] Create additional two phase jpackage tests (revision 3)

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/263/files
  - new: https://git.openjdk.java.net/jdk/pull/263/files/650cf21e..da929c98

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=263&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=263&range=02-03

  Stats: 5 lines in 2 files changed: 1 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/263.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/263/head:pull/263

PR: https://git.openjdk.java.net/jdk/pull/263