Allow to update quotes for several selected securities

[pfalcon]

Some views in the app allow to select multiple securities, so allow to
perform operations on all of them. Without such support, behavior can
be confusing, for example, currently a user can select multiple securities
in "Securities" view, and choose "Update quotes (selected)" via main menu,
but only one will be actually updated.

[pfalcon]

@buchen : I believe in one of your comments here in the issues I saw your comment (was awhile, can't point to now) that supporting multiple selection is somehow challenging. Caught my attention because was one of the initial changes I did. This adds infra for multiple selection, and then specifically implements it for "Update quotes (selected)". Going thru the entire app and making sure it's consistently supported is a different matter of course. Probably why it sat in my fork for awhile. I guess it's better to land this initial support, then none at all.

[pfalcon]

that supporting multiple selection is somehow challenging.

Here it is: #1499 (comment)

[buchen]

I agree. Not sure what made me write the comment 🤔

[myrdd]

Thanks for the PR. I think we should update the command.updateQuotesCurrentSecurity.name text from singular to plural (”selected securities“):

portfolio/name.abuchen.portfolio.bootstrap/OSGI-INF/l10n/bundle.properties

Line 75 in bc41cc6

command.updateQuotesCurrentSecurity.name = Update &quotes (selected security)

This also needs to be changed in all translations.