Fix issue #660

Activate "Don't keep activities" in Developer options to reproduce this bug. When invoking the FormulaEditorFragment from the ScriptFragment it becomes registered with the ScriptActivity as the ScriptActivity is a FragmentActivity. Hence, when leaving the ScriptActivity by entering the stage and returning again by leaving the stage, it will try to resume its fragments. During this process it tries to access formulaTextFieldId of Formula.java. However this field is not available (= null) because it is declared transient. When not declaring the formulaTextFieldId as transient the NullPointerException will be avoided though the FormulaEditorFragment will be loaded anyway. Because all of this only happens when leaving the ScriptActivity via the PlayButton the FormulaEditorFragment is removed beforehand. So if now returning from stage the FormulaEditorFragment is not registered with the ScriptActivity and will not be started by the ScriptActivity.
Fix testReturnFromStageAfterInvokingFormulaEditor

Run testcase testReturnFromStageAfterInvokingFormulaEditor only when 'Don't keep activities' is set.
Merge pull request #680 from Catrobat/CatroidCrashesAfterReturningFromStage

Fix issue #660