Improve status and tag filters

Currently, the user can filter the topics displayed, by selecting another status or another tag in "Status: All" and "Tags: All".

This could be made more powerful:

1) by adding a checkbox symbol on item selected in the selection menus:

=> for instance you could select several tags simultaneously:

to filter topics with "Features" and topics with "Urgent!" (a OR in fact ;-))

Of course, if you click again a selected item in the menu, it would remove the checkbox symbol (2-states)

2) by adding a 3-states symbol on menu items (instead of 2-states suggestion above)

=> by clicking a menu item, then you would get: checkbox symbol, forbidden symbol, no symbol (and so on in a circular way)

The new intermediate state "forbidden" would enable to ignore the selected item in the displayed topics.

All that filters combined would give:

DISPLAY any topics satisfying "any of the checked items" and NOT satisfying "any of the forbidden items".