Add ability to set keybind by listening for a keybind

I know this is neither urgent nor important, but it would be a nice one-time quality of life for any new users of TextToTalk.

I saw that [QoLBar](https://github.com/UnknownX7/QoLBar/blob/master/Keybind.cs) was able to pull off listening for a keybind when the user clicks into the keybind field. That would be a neat feature to add to TextToTalk at some point.

As it is now, users have to use dropdown menus to choose a modifier key and a major key (which is a very long list because it includes all numbers and letters).

With this feature, could see either a text field or a button that indicates it's listening for a keybind, and when the user chooses a keybind and presses it, it saves the keybind in the config settings.