RFR: 8089828: RTL Orientation, the flag of a mnemonic is not placed under the mnemonic letter.

[Ajit Ghaisas]

**Bug :**
https://bugs.openjdk.java.net/browse/JDK-8089828

**Root Cause :**
RIGHT_TO_LEFT NodeOrientation was not considered during mnemonic position calculation.

**Fix :**
Corrected code to consider RIGHT_TO_LEFT NodeOrientation while calculating mnemonic position.

**Testing :**
Existing manual test is modified to test RTL buttons as well.

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Commit messages:
 - rtl_mnemonic_fix

Changes: https://git.openjdk.java.net/jfx/pull/146/files
 Webrev: https://webrevs.openjdk.java.net/jfx/146/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8089828
  Stats: 46 lines in 3 files changed: 37 ins; 0 del; 9 mod
  Patch: https://git.openjdk.java.net/jfx/pull/146.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/146/head:pull/146

PR: https://git.openjdk.java.net/jfx/pull/146