Handle negative numbers in UIUtilities.findDecimal() (Fixes #11484) (rebased onto develop)



This is the same as gh-1571 but rebased onto develop.

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Ticket for reference with more overarching explanation:
* https://trac.openmicroscopy.org.uk/ome/ticket/11484

Test case to be run as follows:

```
./build.py clean build
...
cd components/insight/build/
ant -Dtest.suite.include=**/shoola/util/ui/TestUIUtilities.java test
...
```

Both unit tests should pass.