CheckSezpoz: use better var for APT settings file

There is actually an "org.eclipse.jdt.core.prefs" file in addition to
"org.eclipse.jdt.apt.core.prefs" so it is ambiguous which one a variable
called "jdtSettings" would refer to. The name "aptSettings" is more
clear because AFAIK there is only one configuration file with the text
"apt" in the name, and indeed this configuration relates to the
annotation processing specifically.
CheckSezpoz: split APT properties to constant

This change also removes the hardcoded property key prefix in favor of
inlining it with each element of the constant array. This will be useful
in case there is ever a property that does not begin with that prefix
(hint: see next commit!).
CheckSezpoz: add eclipse.preferences.version prop

Newer versions of Eclipse write this property with a value of 1,
so we include it for completeness.
CheckSezpoz: write APT properties more strictly

This is done because Eclipse 4.3+ removes comments automatically
(particularly the date comment that java.util.Properties#store always
prepends when writing its output). We also want to write the properties
in a very particular order, to minimize the chance of modifications.

See the big comment in this commit for further details.
CheckSezpoz: disallow extension and instantiation

CheckSezpoz is a static utility class, so neither extending it nor
instantiating it makes any sense.
CheckSezpoz: restrict visibility of methods

Methods marked "protected static" are accessible only A) from
subclasses; and B) from other classes in the same package.

Since CheckSezpoz is a static utility class, there will be no
subclasses, so "protected" is actually confusing. We instead use
private visibility for methods not used anywhere else, and default
(i.e., package) visibility for methods which need to be accessible
from CheckSezpozTest (which is in the same package).
CheckSezpoz: remove unused methods

Now that more methods are marked as private, our sometimes-trusty IDE
tells us which methods are never actually used.
Merge pull request #11 from scijava/check-sezpoz-props

CheckSezpoz enhancements