Remove "px" from zero values in CSS

A zero is interpreted the same no matter the units. Saves space.
buttonMarkup: Remove duplicated code
buttonMarkup: Do not allow a value of null to end up the value of an attribute, because it gets stringified as "null"
Docs: Copy editing.
Merge pull request #5655 from jquery/fix-buttonMarkup

buttonMarkup: Do not allow a value of null to end up the value of an ...
Merge pull request #5594 from nschonni/remove-units-from-zero-values

Remove "px" from zero values in CSS
Panel: corrected typo
Support: Turn off pushState inside FF iframes
Panel: CSS for overlay shadow in logical order
Demos: removed superfluous vendor prefixes
Revert "Popup arrow: Convert to demo."

This reverts commit c3866220d520bce9743445e307efb11e0653fc69.

Conflicts:
	docs/examples/popups/arrow.php
Changed .is() to .hasClass() where possible for performance.
Merge pull request #5626 from stforek/duplicated_buttonmarkup

Duplicated code in butonMarkup
Slider tooltip: Converting back to extension.
Demos: fixed broken link to custom responsive grid example
Demos: added collapsing border styling to custom responsive grid example
Demos: improved dynamic controlgroup demo.
Demos: copy editing of panel styling demo.
Demos: made menu title match page title
Demos: cleaned up buttons in grids page