Make default completer complete custom emoji

CMakeLists.txt

man/nheko.1.adoc

resources/qml/Completer.qml

resources/qml/MessageInput.qml

src/CombinedImagePackModel.cpp

src/CombinedImagePackModel.h

src/GridImagePackModel.cpp

src/MainWindow.cpp

src/emoji/EmojiModel.cpp

src/emoji/EmojiModel.h

src/emoji/Provider.h

src/timeline/TimelineViewManager.cpp