Eliminate use of filter_input()

Fixes #74
Change "Update" button to "Preview"

...to help guide users to press the "Save & Publish" at the top of the
customizer.
Change the "Update" / "Preview"button to read "Apply"
Merge branch 'master' of github.com:arnoesterhuizen/wp-widget-customizer into develop

Closes #80
Updating the add-new-widget button to look/work more like a button, with a toggle state.
Merge pull request #86 from shaunandrews/develop

Update the add-new-widget button to look/work more like a button, with a toggle state
Clean up JS to eliminate linter and static analysis warnings
Re-focus last focused element with ID when submitting widget with Enter keypress on input
A wee bit more compact on screens sizes less than 700px tall and more than 981 wide.

Should be cool. Keeps touch target size in tact for tablets/smaller.
Merge pull request #88 from MichaelArestad/develop

Compact widget-tops on smaller laptops, but not tablets.
Clean up some naming, formatting, and commenting
Submit all widget control inputs outside of the .widget-content
Export dimensions, filter is_wide, and if so add class to container
Add slide-out widget panels over preview for wide controls
Construct newly-added wide widgets as wide
Remove erroneous `border-none` from #88
Strip widgets from sidebars_widgets[] settings if not registered

A widget added to a sidebar leaves its widget_id in the sidebar even
when the widget goes away, for example due to a plugin deactivation.
Now any such widget remants will get filtered out when sent to the
client.

Without this fix, a JavaScript error would be thrown:

> Uncaught TypeError: Cannot read property 'transport' of undefined
Collapse any expanded wide widget controls when opening addition panel
Show only one expanded widget control at a time

Prevents abundance of expanded widget controls from cluttering up the customizer.
Solves problem of wide widget controls from appearing over each other.

When one wide widget is expanding while another is collapsing,
the expanding one will always have a higher z-index than the collapsing one.
Eliminate hover/focus styles added to help when multiple wide controls were open

Made obsolete by e7107f583, which only allows one widget control to appear at a time.

Reverts part of 478fb21ff