Cross-widget text select

[emilk]

• Closes Select text across widgets #3816

Turn off with style.interaction.multi_widget_text_select.

There is an API for this in LabelSelectionState, but it's pretty bare-bones.

This became really hairy implementation-wise, but it works decently well.

Limitations

• Drag-select to scroll doesn't work
• A selection disappears if you scroll past one of its end-points
• Only the text of labels and links are selectable

TODO

• An option to turn it off
• An API for querying about the selected text, and to deselect it.
• Scrolling past selection behaves weird
• Shift-click to select a range