fixed blinking to black in rendering window
[bugfix] fixed random crash on closing app with active conversation
added outgoing bitrate estimation to statistics window
[feature] added drag&drop functionality for user list

- user can be added to the current conversation by dragging a user view
from the user list onto the conversation view
[feature] added drop ability to conversation view
[feature] fetch missing streams

- upon adding user to the conversation, check which streams are
currently fetched and fetch only missing ones
[bugfix] fixed random crash when switching b/w streams

- sometimes when switching between several video streams, app was
crashing; fixed it - OpenGL-related multithreaded problem (accessing
OpenGL context when it was not ready yet)
'issues/stability-bugfixes' -> 'dev'