wish: ability to batch-edit selected transactions easily

I sometimes discover new features, or make changes in the way I account things over the years, and want to retroactively correct/standardize things. The problem is that it's not really easy with Homebank currently: you can search and filter very nicely, but you can't change the information in batch, you have to go through each and every transaction one by one to correct them.

Interestingly, as I was writing this, I discovered a section in the user manual that claims there is a (deeply hidden) feature for this, where you shift-doubleclick a column while multiple transactions are selected: http://homebank.free.fr/help/use-tipsandtricks.html#multitxnedit

However,
- It doesn't work. At least, not with 5.0.5; maybe it worked in the GTK2 days. Right now all it does is change the sorting order.
- It is not discoverable. Instead, I would expect to have the normal "Edit" button available/sensitive, and have the transaction editing dialog handle it in a similar way to what Rhythmbox or "ex-falso" do. We can do even better though, and my suggestion is below...

Proposed approach for the multi-edit dialog:
- hide() fields that don't make sense for multi-editing, such as Date, Amount (including the splitting button) and State (reconciled/remind/etc. can be done from the transactions main window, or on an individual basis)
- Instead of writing the text strings directly into the text/comboboxtext entry widgets by default, use https://developer.gnome.org/gtk3/stable/GtkEntry.html#gtk-entry-set-placeholder-text
- For fields where the values are all the same, display the values in the placeholder text; for fields where values differ, write "(varies)"
- When users enter text in those fields, it will naturally replace the placeholder text; when the OK button is clicked at the end, Homebank can then check which fields actually have custom text values in them, and apply those changes.