Improve Handling of "Blank" Dashboard Columns

[kraftbj]

With the new Dash improvements, on wide monitors, four columns exist for dashboard widgets.

If the user is not using all visible, a blank, dotted-borded box is displayed.

This can be confusing to users who don't realize they can drag the widget to those spaces, thinking the dotted lines are placeholders for future widgets.

Ideas for improvements:

• Change the dotted box to some type of note that widgets can now be moved there.
• If not being used, show a smaller blank indicator to maximize the size of the columns being used (A compromise for those who don't want 3 or 4 columns while keeping our fully responsive goal).

[helen]

I agree that this still feels wrong, even though it's been that dotted outline for a few releases now. I also mentioned elsewhere that it could be nice to be smarter about how many columns there are depending on how many widgets are showing - trying to enforce 4 columns when only 2 widgets are even available is pretty lame.

[xavortm]

I added the message "Drag panels here". Please keep in mind its my first ever patch ever, so i any feedback on it would be helpful.

[xavortm]

After having those two changes the string now will be translatable. No DOM elements are being added + it will work on any other empty-container elements. @johnbillion checked it live here at WordCamp Sofia on the contributor's day :) I am not 100% sure if the patch file i attached is properly set, since the first and second one must be applied ;/

[xavortm]

The proper patch file

[xavortm]

Screenshot of the end result

[xavortm]

Minor tweaks for IE7/8 compability

[xavortm]

Minor tweak for IE7/8 compatibility

[helen]

Just a note that they're referred to as "boxes" in help text, not panels.

[xavortm]

From the last patch - updated to "Drag boxes here". Thanks for the note :)

[swissspidy]

Patch still applies and works as expected in Chrome. Needs some testing in other browsers.

[johnbillion]

Tested visually and functionally in Firefox 41, Safari 8, and Chrome 47, IE8, and IE9. IE8 does not show the new message, but remains functional as it is currently.

[johnbillion]

In 36295:

Administration: Add a "Drag boxes here" message to empty dashboard meta boxes so it's clear to users that it's possible to drag meta boxes into empty spaces.

Props xavortm
Fixes #26399

[swissspidy]

In 36296:

Administration: After [36295], mark postBoxL10n as a global to please JSHint.

See #26399.