Update dependency list

[The-Compiler]

Most of the listed dependencies seem to be optional (at least in theory, can't say I've ever tried to run barpyrus outside of hlwm!). Also some more recent changes added some new dependencies.

[preisi]

As there are some optional dependencies listed for a selection of available widget:

• trayer is also required for the trayer widget
• playerctl is required for the playerctl widget

There are probably more, but I'd try to list all optional dependencies.

[The-Compiler]

Thanks! Updated with those for now.