related to #523.
 We were not able to open the simple editor after having saved a unique
symbol area with no stroke. Indeed, the fillLegendMemory attached to
LinePanel was a NullPenStrokeLegend so the init method threw a
IllegalStateException...

The fix is not that clean. We have code where constructors have coupling
with listeners and that produce border effects. I've decided to go as
fast as I could as this design has already been discussed and accepted.
It may change later, however, in order to separate behaviour management
from initialization.

I rely here on a new field that lets me know if the stroke (or fill,
resp.) was originally set so that I can decide if fields must be enabled
or not. It seems to be working, bu I may have missed something, so please
test it.
Fix a problem in PnlProportionalPointSE.
When disabling the stroke of the proportional point, some bad values
were set in the associated Symbolizer. More accurately, we obtained a
Stroke with a null color and that led to a NPE.

I make deeper tests here so that we use the expected objects in both the
legend and the symbolizer. Seems to work, but needs to be tested.
Fix management of mapped legends when the stroke is disabled.
We're now able to deal with it so that, when editing single
configurations or the fallback configurations, the fields associated to
the line configuration are disabled. I've managed to do this by checking
if the stroke is enabled or not in the parent legend. If it is disabled,
I replace the PenStrokeLegend in the temporary unique symbol with a
NullPenStrokeLegend.
Merge pull request #524 from agueganno/issue-523

Management of optional strokes and fills


In some places, it is possible to disable stroke or both stroke and fill. This feature was quite buggy sometimes. #523 illustrates that - trying to open the simple style editor with a style containing a unique symbol area with a null stroke led to an exception. Some other problems I've found while working on it:

    Trying to edit a proportional point with a null stroke led to an NPE.
    Editing the single or fallback values of the mapped legend opens a dedicated editor. The line configuration panel was always active, even when the stroke was disabled.

This PR intends to solve these problems. I've worked in the simple editor in three different places. First, in LinePanel and AreaPanel, I've found a way to be able to build the UIs even if the associated legend element was null. Considering the coupling between these objects and their listeners, I've decided to use an internal field. This gives us stateful objects, but it seems to be working anyway. You'll notice that I disable the fields even when the checkbox is not shown. This way, we can disable the fields when editing mapped legend values.

In ProportionalPoint, I've greatly improved the strategy used to affect the values from the temporary unique symbol to the proportional point. I've isolated the code in a dedicated method in order to improve readability.

In MappedLegend editors for normal and fallback configurations, I've added a test that checks if the stroke is enabled so that we can disable it when needed in the temporary unique symbol legend.

intensive testing is needed here. I may have missed a corner case... Testers need to check that the states are well retrieved when closing the editor and opening it back, for all the mentioned configurations... It may take some time ;-)