RFR: 7903054: View non html files as text

[Dmitry Bessonov]

Added a special check for `.html/.htm` files, to render those containing XML inside (with on-the-fly transformation assumed) as XML in Swing component - which would be as plain text.

-------------

Commit messages:
 - 7903054: View non html files as text

Changes: https://git.openjdk.java.net/jtharness/pull/17/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtharness&pr=17&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903054
  Stats: 24 lines in 1 file changed: 21 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jtharness/pull/17.diff
  Fetch: git fetch https://git.openjdk.java.net/jtharness pull/17/head:pull/17

PR: https://git.openjdk.java.net/jtharness/pull/17