LibGUI: Add an overwrite mode to the TextEditor widget

[cflip]

This adds an overwrite/overtype mode to the TextEditor widget that can be toggled with the insert key. I couldn't figure out how to implement undo/redo, so for now it's just a FIXME.

[stale[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Thank you for your contributions!

[stale[bot]]

This pull request has been closed because it has not had recent activity. Feel free to re-open if you wish to still contribute these changes. Thank you for your contributions!