Add ability to have an icon for dropdown

[grliszas14]

Resolves: audacity/audacity#8576

Allow to set an icon for a dropdown item:

• I signed the CLA
• The title of the PR describes the problem it addresses
• Each commit's message describes its purpose and effects, and references the issue it resolves
• If changes are extensive, there is a sequence of easily reviewable commits
• The code in the PR follows the coding rules
• There are no unnecessary changes
• The code compiles and runs on my machine, preferably after each commit individually
• I created a unit test or vtest to verify the changes I made (if applicable)

QA:

• there should be no visual change in dropdowns in MU4

[Eism]

I am concerned that the dropdown may expand in the future and new properties will be added just because we already have icon property. The dropdown currently looks like a fairly simple component (just show the title), and if you look at the qt combobox component, it also does not have an icon.

Let's explore the approach of customizing the content item for the dropdown, following the example of the content item in FlatButton.