Move definitions from scriptableproxy.h to cpp file
Tests: Added tests for tobase64()/frombase64() and pack()/unpack()
Tests: Added tests for getitem()/setitem()
Tests: Added test for abort()
index() is alias for currentitem()
Tests: Added test for escapeHTMLCommand()
Take default session name from COPYQ_SESSION_NAME

If "copyq" is executed from current session, it will use same session
name (action "copyq add 1 2 3" will add three items to current session).
Tests: Added test for execute()