Issue #2134: Converting context relationship functions into methods.

[quicksketch]

Builds on backdrop#3422 by doing the following:

• Renames "user_from_node" relationship plugin to "author_from_node"
• Moves logic from layout_get_context_from_relationships() to a private method Layout::getContextFromRelationships()
• Builds in loading of context relationships directly into Layout::getContexts() so that relationships do not need to be loaded separately. They can just be requested to be loaded at the same time as all other relationships.

Making this last change required that we be able to differentiate between contexts that are added through relationships versus other mechanisms. This has been a problem with contexts from the beginning: You can't really tell where the context came from other than by a loose naming convention on the array keys (i.e. a integer comes from a menu path, "current_user" is always specially keyed, and now prefixing with "relationship_"). The way the context is added is now explicitly set in LayoutContext::usageType, a public property on the LayoutContext class.

[klonos]

For the sake of curiosity, can someone explain to me why we are using what seem to me exponentially increasing numbers for these constants? What is the trick we're using them for?

[quicksketch]

They are for bitwise math operations, where binary numbers are used to indicate one "flag". These numbers correlate to the following in binary:

1 = 0001
2 = 0010
4 = 0100
8 = 1000

Each number results in a binary number with a single 1 digit and the rest zeros.

Then you can use multiple flags to request multiple types of layout usages by using bitwise math operators, like this:

USAGE_TYPE_CUSTOM | USAGE_TYPE_MENU

Which turns into 0011 (both 1 and 2). That means you can ask for multiple flags in a single argument instead of passing in several arguments, or having a bunch of IF statements that work on strings. This same pattern is used in the menu system in Backdrop, and is common in PHP functions, for example preg_replace_all().

[klonos]

Thanks @quicksketch 🙏🏼

[quicksketch]

I'm continuing final changes in a separate core PR at backdrop#3611. No need to merge this any more.