Change + icon when hovering it with Alt held, like VS Code

Watch this gif carefully. When the + is hovered with alt held, it changes to a split pane icon:


(from https://code.visualstudio.com/updates/v1_57#_terminal-tabs)

We shouldn't change that button every time alt is pressed, just when it's hovered with alt IMO

The issue does not name any files, tests, or entry points. Start by reproducing the terminal tab + button interaction with Alt held and identify the hover handling for that control; done means the button changes to a split-pane icon only while hovered with Alt held, without changing whenever Alt is pressed elsewhere.

Written by the indexing model from the issue text.