[kronolith] restore showing event tag's color

[Pascal Rigaux]

Hi,

Our users liked horde3's feature to assign a color to a category, and  
use it a the background of the events.

Since kronolith now uses the background color to identify the agenda,
we could use a vertical color band, as done in thunderbird lightning  
(cf screenshot)

I've been toying with it, and it renders quite nicely.

WYT?

-- 
Happy new year!

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