Bugfix: Correctly keep preview active during fullscreen-toggle

[smundro]

This is a fix for issues created by #286, as noted in #306.

The initial attempt to keep preview active when toggling full-screen, was to simply not call toggleSideBySide(), which unfortunately didn't account for "no-fullscreen" classes that are applied in the non-fullscreen side-by-side preview mode.

In this PR:

• move to applying.sided--no-fullscreen only to the parent element and using CSS inheritance
• on full-screen toggle, add/remove .sided--no-fullscreen as needed

For future refactoring, it would be cleaner to apply general status classes to the .EasyMdeContainer, such as fullscreen sided and have the sub-elements key off the parent class to avoid having to set classes on multiple elements. This would allow all the .EasyMdeContainer.sided--no-fullscreen ... classes be replaced with .EasyMdeContainer.sided:not(.fullscreen), with similar changes throughout the rest of the code (.editor-toolbar.fullscreen to .EasyMdeContainer.fullscreen .editor-toolbar, etc.).

[smundro]

@Ionaru Anyone using sideBySideFullscreen:false and allowing full-screen toggling will be seeing issues due to my previous change, this fix should be deployed ASAP.

[Ionaru]

It's a big change with lots of code changes, it's going to take a little while to make sure everything still works as it's supposed to.

[smundro]

Understood. Fundamentally, there's only two changes:

1. remove the '.sided--no-fullscreen' class application to child elements : I verified that this doesn't have any side-effects in the code base, the only concern is if anyone using the package is applying their own styles based on the child element classes.
2. applying the '.sided--no-fullscreen' class when toggling full-screen: This was the piece I missed in my original change, when toggling full-screen the classes need to be changed when the side-by-side is open.

I did also just add some add/removeClass convenience functions, because the string manipulation of classNames is a common pattern, but that can be reversed.