Support keyboard shortcuts in text fields

[mtkennerly]

These are already supported by default:

• Paste
• Select all

These would need to be added specially:

• Copy (Clipboard support? iced-rs/iced#295)
• Cut
• Undo/redo

[mtkennerly]

I've implemented these, but I had to make the cut shortcut behave like copy for now. Because of a defect in Iced, cutting could lead to crashes (e.g., select all -> cut -> type any character). Technically, undo/redo can also lead to the same crash, but you'd have to go out of your way to trigger it, so I think it's an acceptable risk for now:

• Delete a character from the end of the text field
• Undo
• Select the last character
• Redo
• Type any character