feat(QBtnDropdown): Support transition props

[xuzuodong]

What kind of change does this PR introduce?

• Bugfix
• Feature
• Documentation
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes
• No

The PR fulfills these requirements:

• It's submitted to the dev branch (or v[X] branch)
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• It's been tested on a Cordova (iOS, Android) app
• It's been tested on an Electron app
• Any necessary documentation has been added or updated in the docs or explained in the PR's description.

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to start a new feature discussion first and wait for approval before working on it)

Other information:

I want to add transition to q-btn-dropdown, but then I noticed it doesn't have related props in spite of inheriting from q-menu. This PR adds transition support to QBtnDropdown

<q-btn-dropdown 
  transition-show="jump-down" 
  transition-hide="jump-up" 
  transition-duration="1000"
>
  <q-list>...</q-list>
</q-btn-dropdown>

[rstoenescu]

Thanks for contributing!

[rstoenescu]

This will go into v2.9.3