Allow user customization of shortcut settings [Enhancement]

[Gnossos]

Some TodoTxtMac keyboard shortcuts conflict with shortcuts used by other software. Because the other software's shortcuts have a certain logic, in terms of terms of both the functions they perform and their consistency with other (non-conflicting) shortcuts the other software uses, the desirable solution is to change the shortcuts in TodoTxtMac. This is a common feature in most software.

Therefore, this is a request to enhance TodoTxtMac by providing an easy way for users to customize the keyboard shortcuts it uses.