Broader minor changes

[bjendres]

1. Let's take the chance for reformatting the affected files according to CiviCRM code style, see Add code quality tools and configuration (PHP Code Sniffer, PHPStan, PHPUnit) #10 for introducing code quality tools. (see Add change processing #9)
2. Rename Message class, e.g. to ShareMessage? -see Add change processing #9 / HERE - see Add change processing #9
3. Let's name the variable $changeProcessingEvent or just $event -see Add change processing #9
4. Let's define the event name in a constant - see Add change processing #9
5. Let's use OOP style for executing API4 actions. - see Add change processing #9
6. Let's rename $peered_node to $peering, as it is not holding the node itself. - see Add change processing #9

[bjendres]

@jensschuppe Please create this minor - but global - changes as individual tickets, I tried changing them in the branch, but then the rebase failed and caused a bunch of extra work.

Let's do this when there's a good moment to do so.