RFR: 8276206: Rename TextBinding class to better express its purpose

[Michael Strauß]

This PR renames `TextBinding` to `MnemonicInfo` and adds the following text to its javadoc:

          /**
    33 +    * Provides information about mnemonics contained within a string.

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Commit messages:
 - Rename TextBinding to MnemonicInfo

Changes: https://git.openjdk.java.net/jfx/pull/678/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=678&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276206
  Stats: 320 lines in 6 files changed: 146 ins; 146 del; 28 mod
  Patch: https://git.openjdk.java.net/jfx/pull/678.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/678/head:pull/678

PR: https://git.openjdk.java.net/jfx/pull/678