darker FormulaEditor delete button

 * fixes #653
 * replaced ```icon_backspace.png``` and
 * ```icon_backspace_disabled.png```
Merge pull request #662 from Catrobat/issue#653

darker FormulaEditor delete button
Fix for issue #618: Exception in ProgramMenuActivity after crash in ScriptActivity or one of its fragments is prevented
Merge pull request #619 from nikwen/issue#618

Fix for issue #618: Exception after crash
Merge branch 'master' into SOR-facedetection
issue SOR-5 playing with OpenGL Video
disabled video playback functionality
issue SOR-6 fixed non-serializable bug