change formulas + "Edit Formula" context menu

+ added click event for every formula
+ open formula editor when clicked on “Edit Formula” in context menu
+ display “Edit Formula” in context menu only when brick has formula
+ bugfixes for BrickCell context menu
Merge pull request #374 from Catrobat/formula-editor-michael

+ onclick event for every formula
+ open edit when clicked on "Edit Formula" context menu
+ bugfixes