Update jump-to-register to popup preview window when idle, as default in newer Emacs.
Only use register preview feature in Emacs 24.4 and above
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Formatting edit.
This change makes the package compatible with both Emacs 24.4 and older. For Emacs 24.4, we get a register preview window.
User can choose to use C-r or not optionally. C-r is enabled by default.