Allow to mask text in paste dialog

This PR optionally allows users to mask the input copy in the paste dialog, for privacy reasons.

We explored and discussed some UI approaches initially, and the “redacted text” technique seemed like the best compromise between simplicity and effectiveness. It’s basically struck-through text with a stroke thickness that matches the line height.
I moved down the keyboard hint, to accommodate the toggle button above the dialog. To me, it would feel justified to style the text as subsidiary (i.e., with reduced font size and slightly diminished color).
After clicking the toggle button, we give focus to the text area, because it seems likely that the user wants that.

tiny-pilot / tinypilot