User variables - quick bug fixes

Fix user variables list adapter error: 
Create a new project, go to scriptActivity, add new "setVariable"-Brick to when project starts, click on "to" field of the "setVariable"-Brick, click on "variables" in formula editor, add new user variable, go back to scriptActivity, assign created user variable to "setVariable"-Brick, press play, return from stage, click on "to" field of the "setVariable"-Brick, click on "variables" in formula editor -> When adding a new user variable, the variable is not shown in list.

Fix user variables stage value error,
Create a new project, go to scriptActivity, add new "setVariable"-Brick to when project starts, click on "to" field of the "setVariable"-Brick, enter 5 in formulaEditor, click on "variables" in formula editor, add new user variable, go back to scriptActivity, assign created user variable to "setVariable"-Brick, go back to projectActivity, press play, return from stage, go to scriptActivity, click on "to" field of the "setVariable"-Brick, click on "variables" in formula editor -> The created user variable has value 0 instead of 5.
(Also occurs when press play in projectActivity or scriptActivity(then press go back one time, in order to force activity change))

For exact error descriptions, look at "testModifyUserVariableValuesInStage" and "testAddUserVarialbeAfterStage" in FormulaEditorUserVariableTest.

Jenkins complete testrun: http://catrobatgw.ist.tugraz.at:8080/job/Catroid-custom-branch-test/543/

Single UiTest for flaky LegoNXTTest: http://catrobatgw.ist.tugraz.at:8080/job/Catroid-single-UI-test/397/