Consolidate all xcb functionality into one subdirectory

The Linux platform has multiple windowing systems. Support for xcb is
currently in progress, support for Wayland is planned in the future. The
way the current xcb support is included is by making some file with a
`_xcb` suffix, and placing `#if PAL_TRAIT_LINUX_WINDOW_MANAGER_XCB` around
most of that file's contents.

With this change, all of the code in AzFramework that uses xcb directly is
inside the `Platform/Common/Xcb` subdirectory. It greatly reduces the
amount of code in compile-time `#ifdef` checks for the chosen windowing
system. It also provides a logical place to include O3DE-specific xcb
C++ wrappers and interfaces, without polluting non-xcb related code.

Signed-off-by: Chris Burel <burelc@amazon.com>

Code/Editor/Core/QtEditorApplication_linux.cpp

@@ -9,7 +9,7 @@
 #include "QtEditorApplication.h"
 
 #ifdef PAL_TRAIT_LINUX_WINDOW_MANAGER_XCB
-#include <AzFramework/API/ApplicationAPI_Linux.h>
+#include <AzFramework/XcbEventHandler.h>
 #endif
 
 namespace Editor
@@ -19,7 +19,7 @@ namespace Editor
         if (GetIEditor()->IsInGameMode())
         {
 #ifdef PAL_TRAIT_LINUX_WINDOW_MANAGER_XCB
-            AzFramework::LinuxXcbEventHandlerBus::Broadcast(&AzFramework::LinuxXcbEventHandler::HandleXcbEvent, static_cast<xcb_generic_event_t*>(message));
+            AzFramework::XcbEventHandlerBus::Broadcast(&AzFramework::XcbEventHandler::HandleXcbEvent, static_cast<xcb_generic_event_t*>(message));
 #endif
             return true;
         }