RFR: 6463708: DefaultButtonModel.setMnemonic generates ChangeEvent for no change

[Prasanta Sadhukhan]

For a DefaultButtonModel, setMnemonic() generates a ChangeEvent even if the new value is the same as the old value (that is, even if nothing has changed). 
Fix is to fire StateChange event only if the value has changed. 

This is consistent with other places like 
https://github.com/openjdk/jdk/blob/master/src/java.desktop/share/classes/javax/swing/colorchooser/DefaultColorSelectionModel.java#L103
https://github.com/openjdk/jdk/blob/master/src/java.desktop/share/classes/javax/swing/DefaultSingleSelectionModel.java#L79
https://github.com/openjdk/jdk/blob/master/src/java.desktop/share/classes/javax/swing/DefaultBoundedRangeModel.java#L309

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Commit messages:
 - Fix
 - 6463708: DefaultButtonModel.setMnemonic generates ChangeEvent for no change
 - 6463708: DefaultButtonModel.setMnemonic generates ChangeEvent for no change

Changes: https://git.openjdk.org/jdk/pull/9536/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9536&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-6463708
  Stats: 64 lines in 2 files changed: 62 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/9536.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9536/head:pull/9536

PR: https://git.openjdk.org/jdk/pull/9536