[Bug TC6.54a] Help file/Define selection

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In the Help file, information about the Define selection dialog is missing (not listed in the Index for example) or has the wrong title depending on how you look at it.

The help section shown when pressing F1 or clicking the Help button is "Find files: General". This could be a combined help page for both Find files and Define selection. In that case the some changes should be made to the help page to reflect that. Also, an entry in the Index named "Define selection" should be added to point to this page.