split flag.inc into different classes

/includes is good; it's what most other modules use.

But I'm afraid this is going to get broken when #871064: Making flaggings fieldable lands. I should have said on this (and the other restructuring issues I've filed) that they're still sort of postponed. I just did a big brain dump yesterday to get these out of my head and into the issue queue. Sorry!

Hmm there's also all those cookie classes I'd forgotten about. I don't think those need a file each. Clearly needs more pondering :/

Here's what I'm thinking:

- keep the cookies classes together in one file: you never use just one alone anyway
- put the flag classes in includes/flag. That's in line with the includes/views pattern (which we don't follow yet, but I'll file an issue for that ;) And also it helps to see which classes are flag types. And it prevents the ugly clash between flag_entity.inc and flag.entity.inc.
- put flag_broken at the bottom of flag_flag (same pattern as views classes again)

Also, this patch may need to ensure there's a clear of the class registry.

We possibly need to do a http://api.drupal.org/api/drupal/includes!bootstrap.inc/function/registr... in an update function. Should cover all our class changes in other issues too.

+++ b/flag.install
@@ -436,3 +436,11 @@ function flag_update_7202() {
+ * http://drupal.org/node/1699752
...
+function flag_update_7204() {

Ah that should be 7300 because we're on the 3.x branch now.

There's also no need to link to this issue. Git blame can produce that if people want it :)

I'm going to test this with a flag 7-2 install (which I need to re-create, as I'd already started changing database tables on my localhost...)

Here's the same patch with the update number changed and the comment tweaked.

I've tested in on a clean flag install and though applying the patch instantly breaks the site (as expected!), going to update.php makes it all work again.

Note: this same update function will cover us for moving the views classes, which I'll do once this is committed :)

One class per file is probably the PSR-0 way. So we'll be doing that for Drupal 8... :)