Display if a variable is local or global

The Variable list now has 2 headers to distinguish between local and global variables.

- added fragment_formula_editor_variablelist_global_headline.xml and fragment_formula_editor_variablelist_local_headline.xml. These .xml files are included in fragment_formula_editor_variablelist_item.xml and used to display the headlines.

- added strings for headlines in strings.xml files in /values and /values-de

- changed the getView method of the UserVariableAdapter, so that headlines are displayed at the correct position

- added testVariableListHeadlines() in FormulaEditorUserVariableFragmentTest.java to test if headlines are correctly displayed