A bunch of small changes from the August club and retro

[danfishgold]

I didn't want to make a PR for each change because they're pretty small so I put them all in a single PR. The commit messages for the complex changes go more in depth.

Changes

• You can mark entire rows
• Text selection doesn't jump when there's an arrow/marker over the mouse
• You can remove markers and arrows on chromium + firefox (oops)
• The code isn't clipped when exporting with a narrow window (not a common use case, I imagine, but still)
• There are keyboard shortcuts for marker/arrow removal (backspace/delete) and deselection (escape)
• Background + text colors are set explicitly

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