wish: enrich the to/from transaction acc combobox

## Context / problem statement

I sometimes deal with 3 dozen savings and chequing accounts spread across at least 3-4 financial institutions, and it gets very tricky to correctly set transfers between accounts in Homebank without either picking the wrong account, or having overly complex and redundant account nicknames.

Homebank already has the group, institution name, and bank account number metadata fields in each account's properties. Therefore, to avoid the need for the user to duplicate all that information into the account nickname, Homebank should expose this extra identifying information into the account picking widgets throughout the application, particularly the transaction editing dialog.

That transaction editing dialog has some sort of ComboBox-like widget (I forget how this particular dropdown widget it's called in GTK) with an editable field, autocompletion, and dropdown arrow to open a popover widget containing a listview of accounts).

## Suggested solution

It would be highly practical and beneficial to show the institution name (when there is more than one institution in active accounts in Homebank) and account number (maybe as 50 or 70% opacity text compared to the account nickname, and maybe with the 4 last digits of the account number in bold or 85% opacity) in that widget and the associated sub-widgets (autocompletion dropdown, and listview popover). There's two ways I could imagine this:

* The extra metadata text being shown "aligned to the right" at the end of the line, or;
* The extra metadata text being shown on a 2nd line below the account nickname (so in effect, this would double the height of rows in the autocompletion widget and listview popover widget, so I'm not sure that would be the preferred option, unless you choose to expose even more metadata, such as account type)

Bonus points if you can also expose a third piece of metadata: the current amount of the accounts. Some banks do this in their web UI, and it is really nice to be able to see not only the account nickname, number, but also the amount, so you know for sure that the proper amounts are going from A to B, and that the debitor account has sufficient funds.

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This would make it sooooooooooo much easier and less confusion-prone to do inter-account transfers in Homebank, or even adding transactions from a blank "Edit transaction" dialog without an account preselected...