OTR plugin works

This set of patches resolves issue #298 on master branch. Changes:
- Removed obsolete callbacks (libotr v3 -> v4 migration)
- Introduced intermidiate routines get_instag() and otrl_context_find_v3() (interface compatible with otrl_context_find from libotr v3)
- Qt4 -> Qt5 migration

Known issues and limitations:
- Fragmentation not working, so using default libotr v3 behavior
- Some maybe useful callbacks not ready (i.e. convert_msg and handle_msg_event)
- Needs more testing