Reserve size of some containers we are filling

src/Cache.cpp

src/ChatPage.cpp

src/CombinedImagePackModel.cpp

src/EventAccessors.cpp

src/ImagePackListModel.cpp

src/InviteesModel.cpp

src/InviteesModel.h

src/MainWindow.cpp

src/MxcImageProvider.cpp

src/ReadReceiptsModel.cpp

src/RegisterPage.cpp

src/SingleImagePackModel.cpp

src/UserSettingsPage.cpp

src/UserSettingsPage.h

src/Utils.cpp

src/encryption/Olm.cpp

src/main.cpp

src/notifications/ManagerLinux.cpp

src/timeline/CommunitiesModel.cpp

src/timeline/EventStore.cpp