Initial compilable version of the libGDX renderer

This commit contains the first, yet untestet version of the libGDX renderer for the Nifty-GUI. The version compiles, how ever its yet not sure if it actually works.
Update libGDX repository url.

Update libGDX repository url in nifty-libgdx-renderer's pom.xml to
obtain the latest nightly snapshots of required libGDX artifacts. The
libGDX repository url in the nifty-libgdx-renderer's pom.xml is actually
still valid, but it contains outdated snapshot artifacts, as the
developers have decided to publish the latest snapshots to a different
repository (see below) after May 26th, 2013. The reason for using a
snapshot is because stable release builds of libGDX artifacts are not
yet available via a public Maven repository (see below for more
details).

From the libGDX Developer's Guide, "Maven Integration" section, found
at:

https://code.google.com/p/libgdx/wiki/MavenProjectSetup:

"Libgdx is currently published to the SonaType snapshot repository.
This will continue to happen until the Maven integration is complete.
The repository only contains the latest snapshot release for now.
Eventually we will publish release builds to SonaType as well.

Add the following to your settings.xml or your project's pom.xml:

  <repositories>
    <repository>
      <id>gdx-nightlies</id>
      <url>https://oss.sonatype.org/content/repositories/snapshots/</url>
    </repository>
  </repositories>"

The nifty-libgdx-renderer pom.xml contains the following libGDX
repository url:

http://libgdx.badlogicgames.com/nightlies/maven

which is the location of an outdated snapshot repository whose artifacts
were last updated on:

26-May-2013 14:46,

almost 3 months ago at the time of this writing, whereas the updated
libGDX snapshot repository at the url:

https://oss.sonatype.org/content/repositories/snapshots/

contains artifacts that were last updated on:

Tue Aug 20 10:18:49 CDT 2013,

today at the time of this writing.
Update libGDX dependency groupId.
Add BatchRenderBackend implementation for libGDX.
Add TimeProvider implementations for libGDX.
Move libgdx renderer module in main parent pom.

Move nifty-renderer-libgdx module in main parent pom so that it is
grouped with all the other rendering modules.
Remove unused code in GdxBatchRenderBackend.
Improve variable names in GdxBatchRenderBackend.
Improve comments in GdxBatchRenderBackend.
Adjust whitespace in GdxBatchRenderBackend.
Comment out temporarily unused code in GdxBatchRenderBackend.
Remove outdated / incorrect documentation in BatchRenderBackend.
Add textureFilename parameter to addQuad method.

In order to create a libGDX-based implementation of the BatchRenderBackend
interface, we need to add a textureFilename parameter to the addQuad
method in the BatchRenderBackend interface because libGDX stores and
retrieves textures from an atlas using a String-based texture name.

This causes a cascade of changes up the call stack in Nifty, adding a
textureFilename or filename parameter wherever it makes sense. Anywhere
that the addQuad method in the BatchRenderBackend interface is called,
the actual filename of the texture image that was uploaded to the atlas
is now passed in.

The reason the filename is being used is because it's guaranteed to be
unique for every texture, as no two images can have the exact same name
and be in the same directory / package.

Also, the other implementations of BatchRenderBackend had to add the new
textureFilename parameter to their addQuad methods as well, although
they aren't used, so no harm is done.

Modify GdxBatchRenderBackend to work with this new parameter, and remove
all the hacks that were necessary to work without it. Also update any
comments related to this change.

Change ImageImpl class name to GdxImage in GdxBatchRenderBackend.
Update documentation in GdxBatchRenderBackend.

Also, use {@inheritDoc} where possible to avoid duplicating javadoc.
Remove 'this' keyword where possible.

This is in order to adhere to Nifty coding style.
Add final keyword to parameters where possible.

Also, change parameter names to match their interface equivalents in the
addQuad method. These changes are to adhere to Nifty coding style.
Break up long line into multiple lines.
Remove camera from GdxBatchRenderBackend.

Remove support for the libGDX orthographic camera in
GdxBatchRenderBackend for now, as it's causing problems and not really
needed.
Rearrange fields and remove useless comment.
Remove unused field.