RFR: 8274854: Mnemonics for menu containing numeric text not working

[Michael Strauß]

This PR fixes an issue with mnemonic parsing by removing the restriction that a mnemonic symbol must be a letter. Now, it can be any character except whitespace.

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Commit messages:
 - fixed mnemonic parsing
 - failing test

Changes: https://git.openjdk.java.net/jfx/pull/647/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=647&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274854
  Stats: 80 lines in 2 files changed: 72 ins; 1 del; 7 mod
  Patch: https://git.openjdk.java.net/jfx/pull/647.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/647/head:pull/647

PR: https://git.openjdk.java.net/jfx/pull/647