wish: Ability to select and export from multiple "Detail" rows at once from the statistics report

[Jeff Fortin Tam]

I often use the nice "Statistics Report" dialog to export filtered transactions across all my accounts to a spreadsheet for further processing/reporting purposes.

However, in Homebank 5.6.3 (running against GTK 3.24.37 on Fedora 38) you can only export "detail" for the selected row from the treeview widget in the upper half. This gets tedious when you have 10+ rows to export.

I would like to be able to select multiple rows (with Shift+clicking for ranged selection, or Ctrl+click for adding/removing individual rows to the selection) and export the details for all of those at once, whether to clipboard or a single CSV file.

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Some related UI suggestions for additional clarity:

* It could be beneficial to rename "Detail to clipboard" more explicitly to "Export selected row's detail to clipboard" and "Export 12 selected rows' detail to clipboard" (depending on the amount of selected rows; otherwise if you want to keep the code simpler you could do with permanently plural form, I guess).

* I prefixed with the "Export" verb because the export toolbar button does not have any tooltip, and generally speaking menu actions should have a verb.

* The "CSV" ones should probably end with an ellipsis ("…") to indicate that further user action (using the Gtk FileChooser) will be required after clicking.

Those are old HIG rules / design patterns that you've probably observed elsewhere too :)

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Tested in Homebank 5.6.3 running against GTK 3.24.37 on Fedora 38.