Improve compatibility with focal_point

Now that we've committed the cropping functionality (via the manualcrop module):

#2173079: Crop pictures when inserting them

I've been doing some in-depth testing of using imagefield_focus and focal_point: the two most popular modules for controlling cropping by selecting a section of the image to "focus" on. My concern was that manualcrop would conflict with them in some way that would create problems for people who want to use those modules (or are already using them) on their Panopoly site.

I couldn't find any problems with using imagefield_focus! It's a per-field thing, so you can easily enable imagefield_focus and NOT enable manualcrop.

However, focal_point will take over every field and (by default) the Media Browser too.

So, I've come up with some minor changes that will make manualcrop and focal_point not conflict:

1. Switched the configuration to enable manualcrop in the Media Browser to defaultconfig, rather than strongarm, and (also using defaultconfig) disable focal_point in the Media Browser by default. This means that, by default, manualcrop will be used in the Media Browser - BUT you can switch that in configuration and it won't complain about the Feature being overridden. This also won't change the configuration of existing sites on upgrade.
2. Added an alter hook that will disable focal_point on any field that manualcrop enabled. So, if you want to use manualcrop rather than focal_point, just enable it for the field. But if you'd rather have focal_point, just don't enable manualcrop and it'll automatically add itself.

I'll commit shortly.