Cleanup Plugin Manager dialog

Merge "Name" and "Description" columns, and move plugin details to tooltips.

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List:
![pm-dialog-1](https://cloud.githubusercontent.com/assets/793526/2841750/2666f90c-d06c-11e3-8bca-f5c98508a97f.png)
Tooltip:
![pm-dialog-2](https://cloud.githubusercontent.com/assets/793526/2841749/2662673e-d06c-11e3-954c-f926557c1393.png)