Introduction of STRING datatype.

* FE-Keyboard update, new String button.
* values.../strings.xml update
* Implementation of Dialog to create strings.
* Adaptation of Internformula classes.
* Interntoken update.
Raw implementation of string functions.

* length of some 'string' function
* lettter x of some 'string' function
* Lint warning fixes.
* String highlighting after user selection.
* Empty String as parameter possible.
* Compute Dialog Refactoring.
* Display of Character returning functions in ComputeDialog.
join() begin of implementation

* Abstract interpretation of function parameters.
Extended implementation for join()
* Test adaption.
* Test and Bugfixes.
* Equal operator for strings.
* Extended test for letter() + bugfix
UserVariable holds an Object now instead of a double.
* Stringfunctions can use UserVariables as parameter.
* The 3 stringfunctions (length(), letter(), join()) can now handle
  parameters of the type UserVariable.
* Implementation of tests.
SetVariable- and ChangeVariableActionTest refactoring and extension.
* ChangeBrightnessByNAction - SetSizeToActionText extension.
* Stringformula interpretation for Change...-Bricks.
* Stringformula interpretation implementation and extension of test:

* GlideToAction
* GoNStepsBackAction
* IfLogicAction
* Lego...Action (no tests)
* MoveNStepsAction
* PlaceAtBrick (only tests)
* RepeatAction
* PointInDirectionActionTest

Stringformula interpretation in BrickAction:

* TurnLeftAction
* TurnRightAction
* WaitAction
* All Set...-Actions
Initial string tests for operators and functions.
Refactoring of Stringparser tests:

* Introducing new FE-Util class.
* Introducing ParserTestOperators class.
* Moving methods to responsible class.
* Rewriting pompous code.

Stringparameter tests for normal functions.

Stringtests for addition, subtraction, division and multiplication.

* Refactoring (Rename of testclasses)
* Testmethod of SensorLoudnessTest moved to SensorTest

General Refactoring of code:

* License Header added.
* Extension of the FormulaEditorUitl Class.
* Removing spam code.
Not a number (NaN) handling in every brick action:

* Formula interpretation to NaN will be handled + test.
* Null-Formula tests.
* Stringfunctionhandling of Not-A-Number parameters.
Note- and SpeakBrick now use FormulaEditor.
Substraction a NUMBER from a Stringfunction caused and uncaught exception.
Prototypeview for Mathfunctions.
Delete of unused resources in strings.xml

Note- and SpeakBrick test update:

* testSoundBrickDefaultValues update
* testComputeDialog update

Fix of lint and checkstyle warnings.
Fix of Checkstyle, PMD and FindBugs warnings.

Update of Formula Editor Keyboard:

* Remove of RAND-button and update of tests.
* Update of STRING-button width.
* Math changed Functions on english keyboard.

Fix of FindBugs warnings.
Concrete exception catching in actions.

* ChangeVariableAction and IfLogicAction return true in act() method
  Formula can not be interpreted.

Refactoring of Formula interpretation methods.
Refactoring of BrickViews with Formula.
Refactoring of Actiontests with Formula.
Fix of 2 failing tests.
Language version upgrade.
Note- and SpeakBrick use new Formula mechanics.

IfLogicAction- and RepeatActionTest: testNullFormula() update.
Fix of some Brickrefactoring related UI-tests.
CatrobatLanguageVersion support. (Note- and SpeakBrick language conversion to FormulaBrick)
Remove of redudant Null-Formula test method (SpeakStageTest.testNullText())
fix for Note and Speak Brick
SpeakAction sprite parameter setter.
Merge pull request #970 from Catrobat/DatatypeString

Formula Editor: Strings
changed imports