Simplify editor settings related to window dimming

Only a single checkbox is now exposed to control whether the editor
window should be dimmed when opening a popup. The main use case
for disabling it is picking colors from the editor window while
a popup is open.

editor/project_manager.cpp

@@ -1730,7 +1730,7 @@ void ProjectManager::_dim_window() {
 
 	// Dim the project manager window while it's quitting to make it clearer that it's busy.
 	// No transition is applied, as the effect needs to be visible immediately
-	float c = 1.0f - float(EDITOR_GET("interface/editor/dim_amount"));
+	float c = 0.4f;
 	Color dim_color = Color(c, c, c);
 	gui_base->set_modulate(dim_color);
 }