Add unread marker on panel button

[unera]

Description

Now panel button can contains counter of tabs that are placed in the panel.

But some tabs can have 'unread' markers (for example if a tab contains a messenger, or mail client)

Example (marker is on pinned tab):

It would be nice if the panel button could inherit unread marker from its tabs.

So if one (or more) panel's tab is unread, then show unread marker at panel button.

[mbnuqw]

It's already implemented for inactive panels, but I can add an option to show that badge in active panel too.