Add a temporary Line to mark the rollback point. Improve UI operability — Slither Link Steam update #408780

1. A Pencil button is added to switch on and off the temporary line. It can be used to mark a key fallback position. The temporary line is different in appearance, but its function is the same as the standard line.Press and hold the Pencil button for 2 seconds to convert all temporary lines to standard lines. The P key performs the same function as the pencil button, except that it cannot be held down. 2. Move the undo, redo, and pencil buttons from the settings to the toolbar, which stays at the top of the screen. 3. Fixed the issue where pressing the left button on the UI and sliding the mouse over the screen would display a preview.