Identify Backdrop version and distinguish it from Drupal 8.x

[quicksketch]

Backdrop right now is using the same constants to define its version information as Drupal 8. This is problematic on multiple levels:

• Backdrop is nothing like API-compatible with Drupal 8, since D8 has changed significantly since our branch point.
• Module developers will need to be able to distinguish Backdrop from Drupal 7/8 modules.
• Backdrop is also not entirely API-compatible with D7 (though it is close). But attempting to work while identifying itself as D7 would cause signficant problems with minor API changes all over the place.

Proposed solution:

• Update the VERSION constant to be 1.0-dev
• Rename DRUPAL_CORE_COMPATIBILITY to BACKDROP_CORE_COMPATIBILITY and set to 1.x
• Set a new property in .info files for "backdrop = 1.x", replacing "core = 8.x".

The new property in .info files leaves open the possibility of making Backdrop-compatible modules, that work on both D7 and Backdrop. We have yet to see if that's actually a real possibility though.

[xiongxin]

great job!

[quicksketch]

I realize I haven't posted about my in-progress work on this issue. I'm preparing a pull request over here: https://github.com/quicksketch/backdrop/compare/117;backdrop-version

Currently it's not passing tests, but anyone interested in picking this up there's a starting point.

[quicksketch]

I finally fixed the tests and updated the PR for our current modules (Aggregator and Toolbar removed, Admin Menu and Views added, as well as a half-dozen test modules).

It seems like generally all feedback has been positive about this suggested change, which would allow dual D7/Backdrop compatible modules. Fortunately it looks like this change is hard-coded enough that even a cache clear does not seem to be necessary. I'd like to move forward on this so we can address the more challenging issue at #3, replacing Drupal with Backdrop throughout code.

[quicksketch]

After a final review and @jenlampton's confirmation, I've merged in backdrop/backdrop#216. Woo. Followups are #3 and #237.

[jenlampton]

Looks like I overlooked one problem with the installer... reopening. PR coming!

[quicksketch]

I merged in the followup at backdrop/backdrop#226 that fixes the issue with the minimal profile's .info file being screwed up.