Refactor LegendsPanel (especially the init method)

Hopefully it is a little easier to understand now.
Oops, I actually needed these methods.

Even though Netbeans finds nothing when I run a "Find usages", these
methods are called by PropertyChangeListeners.

TODO: Find these usages.
Refactor panel methods; move layer tag.

Refactor panel methods to better see the structure of how panels are
added to the card layout. Also put the layer tag centered above the
dialog on the east side -- it is more visible this way.

TODO: Change the layout manager to avoid overlapping. Maybe
GridBagLayout?
Small code reorganization
Rm EPLegendHelper; convert init --> Constructor

The only static method in EPLegendHelper had only one usage, so we just
include it in the LegendsPanel class. Also, it made no sense to me to
use an init method rather than a constructor in LegendsPanel, so I make
this change.
Small cleanup
Initial format of LegendTree
Use a GridBagLayout to fix resize issues.
Rename LegendsPanel -> SimpleStyleEditor

LegendsPanel contains the UI for the Simple Style Editor. It seemed
logical to rename it to correspond.
Rename Edition --> Edit

Fixes #294.
Remove colored borders

This was just to help me understand the layout.
Make doc more concise
Automatically select a newly added Symbolizer

Fixes #438
Using a JSplitPane looks nicer
Fix slow scrolling!

Fixes #440.

It's interesting to note that when I used setUnitIncrement(int) for only
the vertical scrollbar of the JScrollPanel holding a legend (symbol)
panel, the scrolling remained incredibly slow. To be safe, I introduce a
getJScrollPane(ISELegendPanel) method where I set both the vertical and
horizontal unit increments to the constant INCREMENT (=16). This fixes
the problem.
Merge pull request #441 from agouge/refactor/SE

Preliminary refactor of SE code and several UI improvements