Add clear text button to LineEdit

- Add pressed state to clear button
- Enable clear button on all inputs with search icon
- Remove duplicate clear buttons
- Fix rendering of icon for center and right alignments
- Add clear button to more search fields
- Add clear icon to default theme
- Add method to control enabled state of clear button
- Add property to enable clear button from inspector

editor/groups_editor.cpp

@@ -288,7 +288,9 @@ void GroupDialog::_notification(int p_what) {
 			remove_button->set_icon(get_icon("Back", "EditorIcons"));
 
 			add_filter->add_icon_override("right_icon", get_icon("Search", "EditorIcons"));
+			add_filter->set_clear_button_enabled(true);
 			remove_filter->add_icon_override("right_icon", get_icon("Search", "EditorIcons"));
+			remove_filter->set_clear_button_enabled(true);
 		} break;
 	}
 }