[DisplayServer] Add missing `FEATURE_WINDOW_DRAG` flag to Windows, X11 and Wayland display servers.

doc/classes/DisplayServer.xml

@@ -1937,7 +1937,7 @@
 			The display server supports all features of [constant FEATURE_NATIVE_DIALOG_FILE], with the added functionality of Options and native dialog file access to [code]res://[/code] and [code]user://[/code] paths. See [method file_dialog_show] and [method file_dialog_with_options_show]. [b]Windows, macOS, Linux (X11/Wayland)[/b]
 		</constant>
 		<constant name="FEATURE_WINDOW_DRAG" value="27" enum="Feature">
-			The display server supports initiating window drag operation on demand. See [method window_start_drag].
+			The display server supports initiating window drag and resize operations on demand. See [method window_start_drag] and [method window_start_resize].
 		</constant>
 		<constant name="FEATURE_SCREEN_EXCLUDE_FROM_CAPTURE" value="28" enum="Feature">
 			Display server supports [constant WINDOW_FLAG_EXCLUDE_FROM_CAPTURE] window flag.

platform/linuxbsd/wayland/display_server_wayland.cpp

@@ -209,6 +209,7 @@ bool DisplayServerWayland::has_feature(Feature p_feature) const {
 		case FEATURE_SWAP_BUFFERS:
 		case FEATURE_KEEP_SCREEN_ON:
 		case FEATURE_IME:
+		case FEATURE_WINDOW_DRAG:
 		case FEATURE_CLIPBOARD_PRIMARY: {
 			return true;
 		} break;

platform/linuxbsd/x11/display_server_x11.cpp

@@ -152,6 +152,7 @@ bool DisplayServerX11::has_feature(Feature p_feature) const {
 		case FEATURE_CLIPBOARD_PRIMARY:
 		case FEATURE_TEXT_TO_SPEECH:
 		case FEATURE_WINDOW_EMBEDDING:
+		case FEATURE_WINDOW_DRAG:
 			return true;
 		case FEATURE_SCREEN_CAPTURE:
 			return !xwayland;

platform/windows/display_server_windows.cpp

@@ -138,6 +138,7 @@ bool DisplayServerWindows::has_feature(Feature p_feature) const {
 		case FEATURE_SCREEN_CAPTURE:
 		case FEATURE_STATUS_INDICATOR:
 		case FEATURE_WINDOW_EMBEDDING:
+		case FEATURE_WINDOW_DRAG:
 		case FEATURE_SCREEN_EXCLUDE_FROM_CAPTURE:
 			return true;
 		case FEATURE_EMOJI_AND_SYMBOL_PICKER: