GH-1726 better failure detection for updates

Instead of just checking if the new version started, make sure
it is able to write its IPC key to a file and then use the key
to connect to the process.

application/CMakeLists.txt

@@ -85,6 +85,8 @@ SET(MULTIMC_SOURCES
 	MultiMC.cpp
 	BuildConfig.h
 	${PROJECT_BINARY_DIR}/BuildConfig.cpp
+	UpdateController.cpp
+	UpdateController.h
 
 	# GUI - general utilities
 	GuiUtil.h