RFR: 4834298: JFileChooser.getSelectedFiles() failed with multi-selection and double-click

[Tejesh R]

JFileChooser.getSelectedFiles() failed to retrieve files with multi-selection and double-click. This occurs when a single file is selected then enable multi-selection then select the same file through mouse double-click(Two file chooser dialogs used before and after multi-selection enabled). SelectedFiles are updated when a change in files/directory selection is done or when selection is made through keyboard selection. In this case since a single file is selected before multi-selection is enabled and same file is selected again without changing the selection index/directory, leaving selected Files un-updated. 
**Proposed Fix** : Whenever Multi-Selection is enabled check if any files are selected, if yes update the selected Files of JFileChoooser.

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Commit messages:
 - Fix - Updated filepane with fix and added test case
 - Merge remote-tracking branch 'upstream/master'
 - Merge remote-tracking branch 'upstream/master'
 - Merge remote-tracking branch 'upstream/master'
 - Merge remote-tracking branch 'upstream/master'
 - Merge remote-tracking branch 'upstream/master'
 - Merge branch 'master' of github.com:TejeshR13/jdk
 - Merge branch 'openjdk:master' into master
 - Merge remote-tracking branch 'upstream/master'
 - Merge remote-tracking branch 'upstream/master'
 - ... and 2 more: https://git.openjdk.org/jdk/compare/77398430...7129054a

Changes: https://git.openjdk.org/jdk/pull/9996/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9996&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-4834298
  Stats: 106 lines in 2 files changed: 106 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9996.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9996/head:pull/9996

PR: https://git.openjdk.org/jdk/pull/9996