MDL-43862 atto_managefiles: New plugin to manage files

AMOS BEGIN
 CPY [hasmissingfiles,tinymce_managefiles],[hasmissingfiles,atto_managefiles]
 CPY [deleteselected,tinymce_managefiles],[deleteselected,atto_managefiles]
 CPY [unusedfilesdesc,tinymce_managefiles],[unusedfilesdesc,atto_managefiles]
 CPY [unusedfilesheader,tinymce_managefiles],[unusedfilesheader,atto_managefiles]
AMOS END
MDL-44003 editor_atto: Identify the submit buttons in plugins
MDL-44003 Atto: Set type="button" on atto toolbar buttons.

To prevent them accidentally becoming the default submit buttons for the form.
MDL-43861: Atto Plugins: create new plugin for font and background colors
MDL-43861 Atto: Fix for toolbar menu repaint bug in chrome.
MDL-43867 Atto: Accessibility improvements.

1/ Set the aria-labelledby attribute on the contenteditable div (find the label from
original textarea)

2/ Store/restore the selection for the contenteditable div when it is focused. This allows
you to select some text, then go to the toolbar and click a button, and the selection
will be restored before the button effect is applied.
MDL-43867 Atto: Add an accessibility helper plugin.

From testing in all screenreaders, I found that all of their support for contenteditable is not great.
They treat it like a textbox - which means you can type and edit text, but it tells you nothing about
the styles, links or images in the editor. So I added a button to the toolbar, that is only accessible
when navigating via keyboard, that opens an accesssibility helper dialogue. The dialogue shows the list
of current styles, a global list of all links, and a global list of all images. Choosing an image or link
from here, will focus on the editable region, and select the link/image.
MDL-43867 Atto: Add an accessibility checker plugin to Atto.

Checks for images with no alt, images and links with filenames as alternate text/link text, and contrast ratios
less than WCAG 2.0 AA.
MDL-43867 Atto: Fix restore selection on focus

Only do this when the focus is not triggered by a click. This is useful for keyboard
navigation, but not useful if the focus is from a mouse click (you expect the cursor to go to
the clicked position instead).
MDL-43867 Atto: Fixes for the accessibility checker and helper plugins

There were some cut/paste bugs in there.
MDL-43867 Atto: Remove the keyboard hide and seek from the accessibility helper button.

It is actually wrong to hide the screen reader helper button from mouse users because of the voiceover/ios
combination.

We need to support rtl blind touchscreen users!
MDL-43867 Atto: Fix copyright dates and improve accessibility checker

The luminance calculation is now explicitly from WCAG2.0 docs (not relying on YUI). Cleaned up
the report HTML. Remove filename accessibility checks.