Fix scrolling UI of category menu

To fix the category menu scrolling behavior, in addition to preventing the native scroll bar from appearing (https://github.com/LLK/scratch-blocks/issues/1456) and auto-scrolling it when the flyout position updates (https://github.com/LLK/scratch-blocks/issues/1116), we need to make sure that scrolling gestures across all the expected situations will work correctly: trackpad scrolling, mouse scroll wheel, single button mouse, touch (etc?).

Notes from a conversation with @carljbowman @rachel-fenichel @rschamp :

• We should not implement our own custom scroll bars, or attempt to use the blockly workspace scrollbars.
• We can use overflow: hidden to prevent native scrollbars from appearing, and then manually adjust the position of the category menu items by listening for scrolling gestures (trackpad, scroll wheel, touch drag, etc).
• As a followup, when categories overflow at the top or bottom, we can add little buttons to indicate the overflow, which move the category menu by one category when clicked.
• Generally, we will not attempt to make the category menu adjust its position automatically in response to the position of the flyout, to avoid the complexity of bi-directionally managing scroll position state.