Add XSETTINGS support to x11 driver

Import the XSettingsClient implementation to handle the settings
selection.

Currently, we only care about the Gdk/WindowScalingFactor value used by
the windowing system to notify us of display-wide changes in the scaling
factor.

cmake/3rdparty.cmake

@@ -104,7 +104,10 @@ function(get_clang_tidy_ignored_files OUTVAR)
       "hid.m"
       "hidraw.cpp"
       "hidusb.cpp"
-      "hidapi.h")
+      "hidapi.h"
+      # XSETTINGS
+      "xsettings-client.c"
+      "xsettings-client.h")
 
   foreach(SOURCE_FILE ${3RD_PARTY_SOURCES})
     list(APPEND IGNORED_LIST "{\"name\":\"${SOURCE_FILE}\",\"lines\":[[1,1]]}")