Allow hiding specific columns

[SebastianZ]

As for the Net or the Cookies panel the user should be able to hide specific columns. To do so the table header should have a context menu allowing to enable/disable the display of certain columns and to reset the column display.

Test case:

1. Open Firebug on http://www.3site.eu/sessionStorage/
2. Switch to the FireStorage Plus! panel
3. Right-click the table header => A context menu should appear listing checked menu entries for Key, Value, Storage and Scope plus an option Reset Header. (not existing yet)
4. Click the Scope option => The Scope column should be hidden from the table of storage entries. (not possible yet)
5. Right-click the table header again and choose Reset Header from the context menu => All initially displayed columns should be shown again.

Sebastian