Rename "action" handler to "onClickOutside"

[AndreJoaquim]

Closes #45.

I've decided to supersede onClickOutside. This means that if onClickOutside is passed, action is ignored. If both are passed, only onClickOutside is called. If only action is passed, only action is called.

I've added two deprecation warnings:

1. When both onClickOutside and action handlers are passed and are functions, it warns that only onClickOutside should be defined
2. When only action handler is passed, it warns that it's deprecated and onClickOutside should be used instead

I've added and adapted the test suite to include the new behaviours, as well as adapting the README.md

[zeppelin]

Great work, almost clicked the merge button! :) But then it hit me if you could camelcase except-selector (exceptSelector) as well? It'd be nice to remain consistent with the casing of the onClickOutside argument.

[AndreJoaquim]

@zeppelin Totally agree! Will do that today :)

[AndreJoaquim]

@zeppelin Sorry, I've not yet managed to pick it up, but it's not forgotten!

[zeppelin]

@AndreJoaquim no worries, we're not in a hurry ;)

[ondrejsevcik]

hey guys, did you consider using deprecatingAlias for this? https://api.emberjs.com/ember/3.9/functions/@ember%2Fobject%2Fcomputed/deprecatingAlias

I think it may serve its purpose in this case.

[zeppelin]

@AndreJoaquim I've deprecated except-selector in favor of exceptSelector, also removed the action<onClickOutside precedence, because deprecatingAlias (thanks, @ondrejsevcik!) does not allow to have that. In exchange, we have a deprecation workflow more aligned with the rest of the ecosystem.

Thank you for the great contribution!

Next up: element modifiers :)

[AndreJoaquim]

@zeppelin Cool!