First iteration of XML code cleanup

* Mainly renamed variables to conform to our coding standard
* Removed unnecessary 'throws' declarations where possible
* Logic is still the same, code was not touched for the most part
Second iteration of XML code cleanup

* Using static imports in the .xml package to make it easier to read
  (subjectively)
* Removed some hardcoded values and replaced them by constants
* A few more coding style fixes
Third iteration of XML code cleanup

* Added project specific setting to use .* imports when more than 5
  _static_ imports are used in a class.
* Organized imports according to the new rule.
Some fixes. buildProjectPath() 'fixed'

* I made a boo-boo when refactoring
* buildProjectPath() didn't work as I suspected and I made a minor
  mistake when using it. Should be fixed now
Upped targedSdk, removed debuggable, req. OpenGL20

* Increased the android:targetSdkVersion from 15 (IceCreamSandwich) to
  17 (Jelly Bean) so that no compatibility modes are used. Since we have
  a lot of 4.2.1 devices in our project our app implicitly gets tested
  on this API level.
* Removed android:debuggable="true".
  Reason: Since SDK Tools, Revision 8 (from December 2010!) this is no
  longer needed.
  Quote from  https://developer.android.com/tools/sdk/tools-notes.html
  "Support for a true debug build. Developers no longer need to add the
  android:debuggable attribute to the <application> tag in the manifest
  — the build tools add the attribute automatically. In Eclipse/ADT, all
  incremental builds are assumed to be debug builds, so the tools insert
  android:debuggable="true". When exporting a signed release build, the
  tools do not add the attribute. In Ant, a ant debug command
  automatically inserts the android:debuggable="true" attribute, while
  ant release does not. If android:debuggable="true" is manually set,
  then ant release will actually do a debug build, rather than a release
  build."
* Require OpenGL 2.0 ES.
  Catroid now requires OpenGL ES 2.0. Please see http://goo.gl/GzZ7X for
  details. The main reason for us to do this is that Google Play filters
  apps based on - among other criteria - the <uses-feature> elements.
  Since we use libgdx in OpenGL ES 2.0 mode, users without it cannot
  properly use Catroid and the Stage would force close. By adding this
  element, Catroid will not be shown in the Play Store for them, thus
  avoiding this issue.
  Please also see
  https://developer.android.com/about/dashboards/index.html#OpenGL for
  more information about the relative number of devices that support
  Open GL ES 2.0 (at the time of writing, almost 91%).
Removed some static imports

Removed the static imports from our Constants class in the .xml package.
I used static imports for the XML constants within our .xml package,
because in there it - in my opinion, anyway - makes sense and does not
decrease readibility but on the contrary remove some clutter.
Within the .xml package, you expect constants to be related to the code,
which they are. Using static imports for other constants breaks that
concept, I removed them again.
Few more fixed (thanks kaufmaster)
Merge pull request #119 from Catrobat/XMLParserAndSerializerCodeCleanup

XML parser and serializer code cleanup
Merge pull request #121 from Catrobat/AndroidManifestCleanup

Upped targedSdk, removed debuggable, require Open GL ES 2.0
Merge branch 'master' into issue#54