Increase UI commonality between KCM and applet

Summary:
This patch beings the user interface of the applet and the KCM closer together. To
accomplish this, the following changes are made:

• Move the "Default Device" button onto the first row in the applet and make it a ToolButton to reduce its visual weight
• In both the applet and the KCM, show the port chooser UI in combobox form only when multiple ports are available; otherwise show only a text label. This is allowed by the HIG because it is a case of hardware availability (https://hig.kde.org/plattform/settings.html?highlight=inapplicable#behavior)
• Remove the port choosing functionality from the applet's hamburger menu since it's now available in the main UI when applicable

Depends on D24402

Test Plan:
Simple case:

More complex case:

Reviewers: VDG, Plasma, drosca, broulik, sitter, sefaeyeoglu

Reviewed By: sefaeyeoglu

Subscribers: nicolasfella, ndavis, GB_2, mmustac, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24407