Added ability to toggle the visibility of happy-split-views

[Kilowhisky]

As referenced by this feature. #8

[Kilowhisky]

Just found a bug with it to do with glimmer re-applying properties on re-render causing the widths of the containers to get out of whack. Working on it now.

EDIT: this might be a deeper problem since the component is modifying splitPercentage internally and it is also passed in externally. Since it breaks the actions-up data down mentality i can see why the property is being overwritten by glimmer.

[Kilowhisky]

There are two way to solve this problem. I'd like your input on which way is best.

1. The bug can be avoided if the `splitPercentage value is a property on the controller and not just a static value. That way two-way data binding takes over and the value is stored up the tree so when the attribute is re-applied it gets the appropriate value.
2. It can also be solved by manually triggering the mouseMove event anytime one of the splitter panes is re-rendered because of an external property change. This route seems to go against the ember mindset though as it might overwrite an incoming property change.

I'd prefer #1 because it doesn't quite seem to break ember's flow. It does introduce an unexpected gotcha though.

[ZebraFlesh]

I haven't had a chance to review this yet, but based on your description it seems like the ideal solution is to leave splitPercentage as a property that gets passed in externally, but then copy its value for use internally (say, internalSplitPercentage or _splitPecentage). That way consumers can set the value as they see fit and the component would never touch it.

There's a larger unresolved issue of how components communicate with each other (especially in a parent/child relationship) that keeps getting pushed off by the ember team (and rightly so, given the impacts). See the second half of emberjs/ember.js#11170 (comment) for a deeper discussion of that issue.

[Kilowhisky]

I've thought about the internal value holder for splitPercentage but in my tests i was unable to determine if a property reset was caused by the user setting the property manually or by ember deciding to re-render the component. Without that differentiation the component has no idea if it should overwrite its internal value or not.

I know the 'ember-way' of doing things would involve an action fired up anytime the splitter is resized and then a new splitPercentage being driven down. But that's needlessly complicated and breaks the addon's isolation of concerns for no real benefit.

I have been following the parentView / childView saga. Despite what they say they have indeed changed how parentView behaves and its mucked with quite a bit of things. When i was writing this PR i kept having problems where the parentView kept geting redirected to the page instead of the parent component. Very, very, very frustrating.