Allow toolbar popup buttons use bootstrap footer

[dgrammatiko]

Allow custom footer on BS modals rendered from toolbar button

This is a new feature requested at #4563

What it does?

Allows to render modal in the bootstrap official way, where the buttons are on the bottom of the modal window

How to test

Apply this patch and #4563
Change line 83 of administrator/components/com_messages/views/messages/view.html.php to

$bar->appendButton('Popup', 'cog', 'COM_MESSAGES_TOOLBAR_MY_SETTINGS', 'index.php?option=com_messages&amp;view=config&amp;tmpl=component', 600, 250, 0, 0, '', '', '<a href="#" class="btn">Close</a> <a href="#" class="btn btn-primary">Save changes</a>');

You should get:

Mind that the buttons are fake, no functionality is attached

[n9iels]

@test Looks good on desktop, on mobile layout the buttons fall over each other

[peterlose]

Just tested it. It works fine!

[peterlose]

Should we move this RTC? Two good tests

[dgrammatiko]

@zero-24 can we RTC this one?

[zero-24]

Thanks for the bump 😄 RTC now

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_{This comment was created with the J!Tracker Application at issues.joomla.org/joomla-cms/6462.}

[zero-24]

@committer please be sure that #4563 is merged bevor this as this is a requirement.