gio: Add GMemoryMonitor to monitor for low-memory

Add a memory monitor object, with D-Bus and Portal based
implementations. The D-Bus implementation uses the Linux-only
low-memory-monitor Freedesktop project.

Low Memory Monitor D-Bus API:
https://hadess.pages.freedesktop.org/low-memory-monitor/

Android API:
https://developer.android.com/reference/android/content/ComponentCallbacks2.html#onTrimMemory(int)

iOS API:
https://developer.apple.com/documentation/uikit/app_and_environment/managing_your_app_s_life_cycle/responding_to_memory_warnings

Win32 API:
https://docs.microsoft.com/en-us/dotnet/api/microsoft.win32.systemevents.lowmemory?view=netframework-4.8

Tizen API:
https://samsung.github.io/TizenFX/master/api/Tizen.Applications.EventManager.SystemEvents.LowMemory.html

gio/giotypes.h

@@ -122,6 +122,7 @@ typedef struct _GLoadableIcon                 GLoadableIcon; /* Dummy typedef */
 typedef struct _GBytesIcon                    GBytesIcon;
 typedef struct _GMemoryInputStream            GMemoryInputStream;
 typedef struct _GMemoryOutputStream           GMemoryOutputStream;
+typedef struct _GMemoryMonitor                GMemoryMonitor;
 
 /**
  * GMount: