Mac: Fix automatic launch on login under MacOS 13.3.1

[CharlieFenton]

We have always used login items to launch BOINC Manager in the background when a user logs in. This no longer works correctly under MacOS 13.3.1,. This PR implements using a LaunchAgent to do that under MacOS 13 and later.

[CharlieFenton]

This PR is ready to be merged. I will release a hotfix version 7.20.5 once it is merged.

There is also a screensaver bug under MacOS13.3.1 that is widely reported, and apparently has no workaround. If you have multiple displays connected to the computer, the screensaver usually does not appear on the main display. (The bug is intermittent, so it does occasionally appear on the main monitor.) Since the BOINC screensaver uses only the main display, it does not appear at all on Macs with multiple screens. Apple will reportedly fix this in the next release of macOS.

[RichardHaselgrove]

I will release a hotfix version 7.20.5

v7.20.5 already exists as a Linux hotfix - please bump to v7.20.6

And at everyone - not just you, Charlie - can we please have a concentrated effort to get v7.22 working, and properly, this time?

[AenBleidd]

And at everyone - not just you, Charlie - can we please have a concentrated effort to get v7.22 working, and properly, this time?

This is exactly what I'm gonna work on after the hotfix for MacOS