[Applet]Hide hamburger menu when empty

Summary:
Don't show devices/applications hamburger menu when there are no options.
Hide "Record/Play all audio via this device" if there is only 1 device.
Prevent unavailable ports be active.

Last is a workaround to prevent locking a device to an unavailable port.
This can happen cause of the lack of a call from pulseaudio for a port
availability change.

Depends on D26418

Test Plan:
Lock can happen when an unused and available port becomes unavailable.
Until data refresh, a user can select the unavailable port. After the
switch if there is only 1 available port, UI will hide port selection,
locking user to an unavailable port.

2 vs 1 output devices
{F7876434}

{F7876435}

Reviewers: #vdg, #plasma, drosca, ngraham

Reviewed By: #vdg, ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26574