Update checking feature (#2648)

* Check on startup (toggleable setting) and manually
* Option to check for pre-releases (eg, 2.4.0-beta1)
* Only included if WITH_XC_NETWORKING is enabled

CMakeLists.txt

@@ -61,9 +61,9 @@ if(WITH_XC_ALL)
     set(WITH_XC_YUBIKEY ON)
     set(WITH_XC_SSHAGENT ON)
     set(WITH_XC_KEESHARE ON)
-  if(APPLE)
-    set(WITH_XC_TOUCHID ON)
-  endif()
+    if(APPLE)
+        set(WITH_XC_TOUCHID ON)
+    endif()
 endif()
 
 if(WITH_XC_KEESHARE_SECURE)