Handle hotkey conflicts

[steinbro]

Three notable changes:

1. Use the single-instance crate to check if the program is already running. If so, display a dialog and exit.
2. If any hotkeys fail to bind, such as due to a conflict with another application, leave the action unset but continue running.
3. Do not suppress println messages if the executable is run from a terminal window. Very useful for debugging.

Fixes #13 and #27.