Solve issue #492.
The nullity check is not enough anymore since we've tried to avoid null
values in PenStroke fields. We must check that the dashArray is not
empty before trying to find double values in it. If we do not, we'll
obtain an NumberFormatException.
Fix a problem with the apply button in the advanced editor.
It replaced the list of available styles, in the TOC, with a new list
containing only the style being edited currently.
First part of a huge refactor
Remove local getFallbackSymbolizer methods
Replace line UOM comboboxes with LineUOMComboBox
Introduce LinePanel et. al.

Again, we're working on modularizing panel initialization in order to
solve #484. Here we introduce the LinePanel class which necessitated the
creation of four other classes:

    * ColorLabel (to be reused in future commits)
    * DashArrayField
    * LineOpacitySpinner
    * LineWidthSpinner

As for #497, I've added a local preview, but the area settings don't
update it yet. I'll be fixing this in a future commit.
Introduce USPanel; Fix PnlUP constructor
Introduce AreaPanel; fix a few usages

This introduces AreaPanel. In addition, there were a few calls to an old
constructor of PnlUniqueAreaSE and PnlUniquePointSE that needed to be
updated.

This also fixes #497.
Add missing license headers
Remove confusing penStrokeMemory from LinePanel
Remove some confusion about fill memory
Small refactor for setting fields' state
Push optional part up to superclass
Make init methods override an abstract init()
Push "Enable" checkbox up to superclass
Introduce PointPanel et. al.

Continuing with the modularization to fix #484, here I introduce
PointPanel, which necessitated the creation of several other classes.
Add mouse wheel scrolling to spinners.

Fixes #501. Note: This fix only applies so far to the parts of the
Simple Style Editor that have already been refactored. (Still left to
do: Proportional Point / Line.)
Add missing LookupFieldName interface
Add missing InterpolationLegend interface.
Introduce ProportionalPointPanel et. al.

Also

    * FirstValueTextField
    * SecondValueTextField