Add some missing a11y labels in the keyboard panel

This MR primarily adds a bunch of accessibility labels throughout the keyboard settings panel. It also adds mnemonic widgets in the edit shortcut dialog. Note that i'd like to fix the input source radio button which is not a radio button but two list rows with checkboxes, but i don't know how to handle these without duplicating the row's label in the a11y labels for the checkboxes (adding a labelled by relation for the list row does not work as the list row is labelled by the label and the labelled by relation is not resolved recursively).