Sync to the master
set comics sans font in the student interface,
margins of the login form are now increased,
green/red color for correct and wrong answer,
focus effect in the teacher interface is now implemented
intermediate commit
first commit with text selection

added the facility to highlight text in continuous and discontinuous
mode.
modified buttons in the teacher interface.
added the menu for the selection of grammar categories in the teacher
interface.
modified the "complex exercise" function in the front end and in the
backend in order to send  the grammar category from the teacher
interface to the student interface.
commit teacher selection

added teacher selection function
added touch support in the student interface
started answers communication partially implemented
commit sync with master

updated py files with the original files in the original master branch
modified the py files with only the changes able to support the new
features
commit working version with selection exercise
commit minor bug fixed
Fix conflicts to merge