wish: Filter templates by current account

[rugk]

IMHO there should be a way to easily filter the templates, when selecting them from the form to add a transaction by the account they have assigned to it.

## Problem

My use case is just that I often open an account, add transaction and then add a template. In this state HomeBank could be so "clever" to already know that I am likely wanting to use a template for this account.
Actually I had the case that I accidentally selected a template for another account and then wondered why it did not show up in this account I have open. So this feature would also prevent such mistakes.

If one opens the main "add transaction" form from the main window, that of course does not matter and the settings can be hidden there.

## Solution

This can be quite easily accomplished with a checkbox next to the "Show planned" one.

IMHO you should even enable this feature by default, as, as explained above, it is a "clever" (i.e. intuitive) behaviour that makes quite much sense. As you may leave the user an option to select/deselect the checkbox they can still have the old behaviour back.

The only question may be what to do with templates without an account. IMHO you may just display these too, when the filter is enabled, as these templates of course may be used with multiple accounts, so you do not know whether they should be filtered.

As a concrete example:

Checkbox (default state)
* [ ] Show all accounts

-> only shows the templates for this account

Checkbox (default state)
* [x] Show all accounts

-> shows for all accounts

[Maxime DOYEN (mdoyen)]

done