TerminalDisplay: Limit mouse move events.

The terminal display mouse tracking is based on character boxes, i.e., unless the mouse moves out of a character box its position hasn't really changed. So most of the logic in TerminalDisplay::mouseMoveEvent can be skipped/omitted till the pointer crosses box boundaries.

Before my changes applications like Vim/Neovim couldn't do selection via dragging with the left mouse button (there might be a bug there). I cursorily checked other features like the color filter and URL underlining, and they work.