RFR: 7903053: Open in browser button

[Dmitry Bessonov]

Adding a button in the Files navigation panel to allow launching external browser or other associated app to display the selected file (.html or .java etc.)

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Commit messages:
 - 7903053: Open in browser button

Changes: https://git.openjdk.java.net/jtharness/pull/16/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtharness&pr=16&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903053
  Stats: 27 lines in 2 files changed: 26 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jtharness/pull/16.diff
  Fetch: git fetch https://git.openjdk.java.net/jtharness pull/16/head:pull/16

PR: https://git.openjdk.java.net/jtharness/pull/16