Fixed issues with slider focus and scroll input

(cherry picked from commit 707cf278a517aac9718b6322a32208085c322999)

scene/gui/slider.cpp

@@ -74,8 +74,10 @@ void Slider::_gui_input(Ref<InputEvent> p_event) {
 			}
 		} else if (scrollable) {
 			if (mb->is_pressed() && mb->get_button_index() == BUTTON_WHEEL_UP) {
+				grab_focus();
 				set_value(get_value() + get_step());
 			} else if (mb->is_pressed() && mb->get_button_index() == BUTTON_WHEEL_DOWN) {
+				grab_focus();
 				set_value(get_value() - get_step());
 			}
 		}