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

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.