Only show arrow menu indicator when menu exists
In MenuButtonPopupMode a tool button will have an area that can be clicked to open a menu instead of triggering the primary action of the button. This area with an arrow indicator shows up when a menu is added. Before this commit, when that menu is removed later, the arrow would still stay and serve no purpose while doing so. Even worse: It can confuse users who click on that arrow area and then nothing happens.
This commit improves this by not showing the arrow indicator area anymore after the menu is removed.
Unfortunately, this commit is not perfect because the width of the button stays slightly too wide after removing the menu.
This change hasn't been tested recently. (Should still be fine though) It was extracted from my comment over here and is a step towards system/dolphin!528 (merged).