Finally, port the tagging feature to KDE4. It is in a slightly worse state than what I had for KDE 3.5, but since the difficult part is over, I hope I will improve it over time.

Currently it has the following features:
- Ability to tag emails (with possibly more than one tag)
- Changing the appearance of message headers (font and text color) according to the message's tags.
- Configuration of tags through a new tab called "Message Tags" on the "Configure KMail->Appearance" configuration page.
- You can add toolbar buttons for easy tagging.
- You can assign shortcuts to tags.
- Tagging through the right-click menu.
- Quick filtering using the combobox for "status" filtering. See the screenshot.

Some short-term todos:
- Incorporate Nepomuk
- A column for tags in the header view
- Incorporate Ellen's usability comments

Some long-term todos:
- Filter action for automatic tagging
- Searching based on tags ( which enables having 'tag folders' through use of search folders )

I'm sorry that I didn't have enough time to test my current commit completely. In the last two days, I have been constantly compiling kdelibs, kdepimlibs, kdebase to keep up with very frequent changes. I finally was able to compile all at once:)