Cleanup Element class.

The Element class in nifty-core is badly in need of some basic cleanup.

Remove useless, tautological comments/javadoc. Add some useful
comments/javadoc where necessary. Improve comment/javadoc grammar,
sentence structure, and spelling.

Rename getElements to getChildren, add to addChild, and the overloaded
add method that takes an index parameter to insertChild because those
methods names are infinitely clearer. Rename these methods in all
calling classes. Also rename elements field to children.

Recreate the two old add methods, but mark them as deprecated and
delegate to the new methods.

Fix code style violations so that it matches the code style of the rest
of the project.

No actual logic is altered. All tests are passing, and
integration-testing reveals no problems.
Fix NullPointerException in Nifty class.

The Bug: If Nifty.exit() is called more than once in a row, or if the
current Screen is an instance of NullScreen when Nifty.exit() is called,
then a NullPointerException is thrown in the current Screen's
endScreenMethod, specifically when it tries to call
Nifty.resetMouseInputEvents() on a null instance of Nifty within the
resetLayers() method, since the Nifty instance will be null if the
current Screen is an instance of NullScreen.

The scenario above specifically occurs when exiting from the
AllExamplesMain example demo from nifty-examples when running the
example from nifty-examples-libgdx. When the "Exit" menu choice is
clicked in the menu scrollPanel, and the button labeled "yes" is
clicked, Nifty.exit() is called, but the main application window itself
stays open. The NullPointerException described above occurs when closing
the main application window because that in turn calls
LibgdxExampleApplication.dispose(), which in turn calls Nifty.exit(),
like it should because it can't assume that Nifty.exit() has already
been called.

The Fix: Don't call the endScreen method in Nifty.exit() if the current
Screen is null, i.e., an instance of NullScreen. In fact, since
Nifty.exit() is designed to only ever be called once, and since it
should really be idempotent since that increases robustness, simply
return immediately from Nifty.exit() if the current Screen is null.
Since the first invocation of Nifty.exit() will always make the current
screen an instance of NullScreen, this is effectively the same as
returning immediately if Nifty.exit() has already been called.

Add a test to ensure that calling Nifty.exit() multiple times in a row
does not throw a NullPointerException.
Clean up ScreenTest & upgrade EasyMock.

Upgrade EasyMock dependency in main pom.xml to version 3.1.

Change EasyMock imports from package org.easymock.classextension.* to
org.easymock.*, since the classextension package is deprecated; i.e.,
the functionality has been merged into standard EasyMock in version 3.1.

When stubReturn-ing the getMsTime() method on the TimeProvider mock,
specify the return value as 0L to satisfy the compiler, since it needs
to return a long, rather than casting the 0 to a long, since it's more
elegant.
Minor cleanups.

1) Remove useless, incorrect, outdated comment from Nifty constructor.

2) Remove strange, invalid @created javadoc from
SpecialValuesReplaceTest. It creates a warning when running mvn site on
nifty-core.

3) Rename android module in nifty-examples-libgdx from 'Main Android' to
'NiftyExamples for LibGDX - Android' so that it's clear when building
Nifty with the maven reactor that the module is a submodule of
nifty-examples-libgdx.

4) Rename core submodule in nifty-examples-libgdx from 'Main Core' to
'NiftyExamples for LibGDX - Main Core' so that it's clear when building
Nifty with the maven reactor that the module is a submodule of
nifty-examples-libgdx.

5) Rename desktop submodule in nifty-examples-libgdx from 'Main Desktop'
to 'NiftyExamples for LibGDX - Desktop' so that it's clear when building
Nifty with the maven reactor that the module is a submodule of
nifty-examples-libgdx.

6) Create a logging.properties file for nifty-examples-libgdx, and
configure all logging to Level.WARNING. Remove the static Logger
instance from LibgdxExampleApplication and use a LogManager to load the
logging.properties file instead.

7) Fix an indentation error in nifty-examples-libgdx parent pom file.

8) Fix code style violation in GdxInputSystem by removing unnecessary
uses of the 'this' keyword.
Add displayItems method to DropDownBuilder.
Merge pull request #153 from 3xp0n3nt/element-class-cleanup

Cleanup Element class.
Merge pull request #155 from 3xp0n3nt/nullpointerexception-fix

Fix NullPointerException in Nifty class.
Merge pull request #156 from 3xp0n3nt/cleanup-screentest

Clean up ScreenTest & upgrade EasyMock.
Merge pull request #157 from 3xp0n3nt/minor-cleanups

Minor cleanups.
Merge pull request #158 from 3xp0n3nt/update-drop-down-builder

Add displayItems method to DropDownBuilder.