gdu-format-disk-dialog: Tweak the "Erase Existing Data" option

"Erase Existing Data" is misleading, because it can induce the user to think that their data won't be affected if this option is off. Also, this string doesn't properly explain that data will be overwritten.

So, rename the option to "Overwrite Existing Data" and add an info popover explaining about this option, just like in GduFormatVolumeDialog.

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