Design all interactions for customizing dashboard contents

Dashboard content

We need to redesign blocks for the "What's new" pattern: content, comment, people. We'll also want a search block shown by default. Then maybe a specific block for 'Update status'.

Issues:
priority: #337947: Correct whitespace issue for 'recent content block' in dashboard
#614418: Redesign 'Recent comments' block for use on dashboard.
#614410: Patterns for recent-items-blocks on the dashboard
#569190: D7UX: Dashboard contents
#614414: Revisit 'Who's new' block for use on dashboard

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Customization workflows

Drag and drop

* !! customizing a block to not show it's title makes it nearly unusable in the dashboard. (I suspect that playing around with title/no title also triggered a warning "The block Powered by Drupal was assigned to the invalid region -1 and has been disabled." after clearing caches)
* Not obvious the widgets are draggable. needs design of hover/active states
* Dragged widget leaves visible ghost in original position
* Regions as dropzones, dashed border clash with contextual links hilite
* The drop 'shadow' of where it will land in the region needs to look different from the actual widget being dragged
* Once dropped, should show some indication of the loading process before the actual block contents show
* Allows re-ordering within the tray as well, not sure that's a good thing
* It's unclear how you can remove blocks from regions (drag back to tray).
* Dragging blocks back to the tray is a very hit-miss experience, can't tell why sometimes it will, sometime won't take.
* Can't seem to remove the 'management' block from it's region. This might be by design?
* Clicking customize should show some 'loading' indicater, and already show (something the size of) the tray to keep the screen from jumping up and down.

Issues:
#633086: Dashboard visual design improvements

Excluding/including block availability for dashboard

* The current 'add a custom block' link is too generic. I expect to choose other existing blocks, not create a new one from scratch
* Need a place to manage that list of blocks:
- 'dashboard' region on blocks page?
- per block a checkbox 'make available for dashboard'? (might work if default setting is 'No').
- checkbox on blocks page, à la bulk operation? (no)

Issues:
#601932: Allow dashboard to limit available blocks

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Visual design

* Widget tray is too heavy grey
* Layout of widgets vs. Done button in the tray is weird

Issues:
#633086: Dashboard visual design improvements
#628740: dashboard block storage area is messy

The big one that implemented the initial commit: #544360: D7UX dashboard module

#628740: dashboard block storage area is messy
#614418: Redesign 'Recent comments' block for use on dashboard.
#337947: Correct whitespace issue for 'recent content block' in dashboard
#607324: Empty dashboard blocks unexpectedly sink to the bottom without a notice

#646762: Blocks titled <none> unusable in dashboard