added mathematical functions and clear button

added menu to select mathematical functions and clear/backspace button
in UITextField
FormulaEditorTextView auto-resizing

Changed UITextField to UITextView, added auto-resizing and auto-scroll
to bottom.
Added logical operators
minor layout improvements

+ added border to formula in ScriptCollectionView
+ limited length of formula in ScriptCollectionView
+ removed trailing whitespace in FE textfield
Merge pull request #383 from Catrobat/formula-editor-michael

minor layout improvements