Added submenuOptions property to Dropdown

[spikyjt]

A submenu options array adds flexibility to the dropdown, allowing custom attributes to be added to submenus. This patch doesn't break BC, as it sets submenuOptions to options, if it has not been explicitly set. However it does get the rid of the dropdown-menu class that is currently set on submenus, causing them to be hidden by default.

[cebe]

You have a failing test here: https://travis-ci.org/yiisoft/yii2/jobs/52027817#L917 can you please fix it?

[spikyjt]

Looks like that fixed it. Sorry, I'm new to unit tests. They're still on my todo list to get familiar with!

[samdark]

Test doesn't look right now.

[spikyjt]

@samdark do you think that the submenu should have the class dropdown-menu? Having this class is incorrect as far as Bootstrap is concerned. You can't have a dropdown inside a dropdown. You can have a submenu, Bootstrap just doesn't provide styling for them by default. Removing the dropdown-menu class from the submenu makes it behave as expected, i.e. shown by default with the parent dropdown, instead of being hidden with no way to show it.

[samdark]

Ah, then it's probably fine.

[spikyjt]

Thanks @samdark . The test is definitely now correct as far as my code change is concerned. As long as you are all happy with the logic of my code change, then all is well.

[spikyjt]

Should I add to the test to include settings for the new submenuOptions property?

[samdark]

Yes, that would be great.

[spikyjt]

Ok, my new test passes, but I'm wondering if I should make submenuOptions a property of each item, rather than the whole widget. That way you could set it differently per level, if required. Perhaps I should have created an issue for this rather than just a PR?!

[klimov-paul]

@spikyjt, can you please resubmit this PR up to yii2-bootstrap repo?

[klimov-paul]

Migrated to yiisoft/yii2-bootstrap#41