Revert "Integrate Backup Center into preferences."

This reverts commit dfa5b9f1d81d99703198df1eaa1a7e858ec73e8f.
[#179] reset global shortcut immediately after change

apparently the dot in the preference key name breaks the KVC observer
that monitors the changes and resets the shortcuts... ಠ_ಠ
[#179] disable preferences if wallet is locked
[#179] ignore shortcuts if wallet is locked
[#179] minor copy/layout tweaks in shortcut keys panel