Update status bar icon color for BigSur

[Lweek]

• from 10.14 there are dynamic colors available to reflect system colors.

Related to Issue 542:
#542

[CLAassistant]

All committers have signed the CLA.

[rickygao]

Looking forward to the merge of this pr.

[Lweek]

It would be cool to try it out on older macOS before merge.

There are two suitable colors for this. LabelColor and ControlTextColor. In past on Catalina I have used .controlTextColor and it worked like charm but on BigSur this color becomes less contrast and it is slightly lighter than I would love. Not horrible but either not perfect. There is no official word about what color is the right one so I experimented and .labelColor fit best for BigSur.

I will try to reach Apple for answer but it will take some time and I think this solution is just fine for now. :)

[rickygao]

As is discussed in #504, isTemplate being true may be a better solution?

[Lweek]

Well. You draw unread message counter on it and so I think you shall not use isTemplate because it would probably tint the counter as well? Dunno. Somebody should give it a try. :)

[Skyearn]

Well. You draw unread message counter on it and so I think you shall not use isTemplate because it would probably tint the counter as well? Dunno. Somebody should give it a try. :)

How about determine the status bar icon according to the system version? Below 10.14 use .controlTextColor and .labelColor，isTemplate being true in 10.15 or higher.