issues with Strings being compared with either == or !=

[scuniff]

The Static Code Analyzer Spot Bugs found the following issues with Strings being compared with either == or != and should be using equals().

From Spot Bugs:

Comparison of String objects using == or !=

(https://spotbugs.readthedocs.io/en/latest/bugDescriptions.html#ES_COMPARING_STRINGS_WITH_EQ)
This code compares java.lang.String objects for reference equality using the == or != operators. Unless both strings are either
constants in a source file, or have been interned using the String.intern() method, the same string value may be represented by two
different String objects. Consider using the equals(Object) method instead.

Bug kind and pattern: ES - ES_COMPARING_STRINGS_WITH_EQ

Occurrences:

https://github.com/imagej/ImageJ/blob/master/ij/gui/PlotContentsDialog.java

if (allTableNames[i] == previousTableName)

https://github.com/imagej/ImageJ/blob/master/ij/plugin/SimpleCommands.java

if (label2!=label) {

https://github.com/imagej/ImageJ/blob/master/ij/macro/FunctionFinder.java

if (exists=="0") {

https://github.com/imagej/ImageJ/blob/master/ij/plugin/frame/Recorder.java

if (commandOptions==null || commandOptions==" ")

occurs twice