Editor: Dim UI when a WindowDialog is shown.

Darkens the editor on WindowDialog popup.

This adds the following new Editor settings:

- interface/dim_editor_on_dialog_popup (true) # Enable/Disable editor dimming
- interface/dim_amount (0.6) # Percentage of how much the editor will be darkened (0-1)
- interface/dim_transition_time # The duration (in seconds) of the color blending effect (0-1), 0 is instant.

Please test this thoroughly, I haven't yet seen a case where it fails to work properly but I'm sure I didn't test all
windows of the editor :P

scene/gui/dialogs.cpp

@@ -31,6 +31,10 @@
 #include "print_string.h"
 #include "translation.h"
 
+#ifdef TOOLS_ENABLED
+#include "editor/editor_node.h"
+#endif
+
 void WindowDialog::_post_popup() {
 
 	drag_type = DRAG_NONE; // just in case
@@ -199,6 +203,16 @@ void WindowDialog::_notification(int p_what) {
 					set_default_cursor_shape(CURSOR_ARROW);
 			}
 		} break;
+#ifdef TOOLS_ENABLED
+		case NOTIFICATION_POST_POPUP: {
+			if (get_tree() && get_tree()->is_editor_hint())
+				EditorNode::get_singleton()->dim_editor(true);
+		} break;
+		case NOTIFICATION_POPUP_HIDE: {
+			if (get_tree() && get_tree()->is_editor_hint())
+				EditorNode::get_singleton()->dim_editor(false);
+		} break;
+#endif
 	}
 }