Add jdenticon support

CMakeLists.txt

resources/qml/Avatar.qml

src/JdenticonProvider.cpp

src/JdenticonProvider.h

src/MainWindow.cpp

src/MainWindow.h

src/UserSettingsPage.cpp

src/UserSettingsPage.h

src/timeline/TimelineViewManager.cpp

src/timeline/TimelineViewManager.h

src/ui/Theme.h