Implement image uploads (#24)

CMakeLists.txt

include/ImageItem.h

include/LoginPage.h

include/MainWindow.h

include/MatrixClient.h

include/TextInputWidget.h

include/TimelineItem.h

include/TimelineView.h

include/TimelineViewManager.h

include/ui/CircularProgress.h

include/ui/LoadingIndicator.h

src/ChatPage.cc

src/ImageItem.cc

src/LoginPage.cc

src/MainWindow.cc

src/MatrixClient.cc

src/TextInputWidget.cc

src/TimelineItem.cc

src/TimelineView.cc

src/TimelineViewManager.cc