MDL-39851 javascript: Full screen dialogs on small screens

This change adds support to M.core.dialogue for showing fullscreen
dialogs on small screen sizes. There are 2 new configuration values that
can be set to control this when creating a dialog (responsive and responsiveWidth).

Setting responsive to false will disable this functionality completely (old behaviour).
Setting responsiveWidth controls the screen width that the dialog will switch to
fullscreen mode. Try not to think of the device that will be accessing the dialog,
but the width at which the content of the dialog needs to be displayed differently.

The activity chooser and the filepicker are the two dialogs that have been converted
in this patch. The filepicker first had to be converted to use M.core.dialog and not
Y.Panel.
MDL-39851 forms: Convert filemanager popups to M.core.dialogue
MDL-39851 filepicker: Fix for toolbar display wrapping on small screens
MDL-39851 M.core.dialogue: Various fixes

Items addressed
* Get rid of custom attribute srcNode
* Change extraClasses to a config only parameter.
* Change attribute fullscreen to a method.
* Restore the original window overflow value.
* Use setStyles instead of setStyle for performance.
* Tinymce end the zindex wars
* Prevent fullscreen dialogs in an iframe (tinymce managefiles plugin).
MDL-39851 tinymce: Cross browser fix for zindex on popups

The previous solution was only working on chrome because it was
relying on "win.name".
MDL-39851 M.core.dialogue - More changes to fullscreen behavior.

Use CSS for overflow changes (easier to keep track of the previous state).
MDL-39851 M.core.dialog : Coding style for css changes