Mail-Status and other Tracker add-ons should behave consistently and offer configuration UI flow

[grexe]

We need to establish developer guidelines so Tracker and its add-ons behave consistently. Haiku is not even at R1 and we already have divergent UI which is not good.

Case in point: add-ons configuration.

Tracker's "New" templates can be configured right from the menu. Same for WebPositive bookmarks. There is (and should be!) always a direct link to configure what is being shown.

However, the Mail status Tracker-addon does _not_ offer any configuration in the menu, and I could not find any place to configure the various status labels. This should be fixed.

[humdinger]

The "Mark as…" addon should probably show a pop-up like:

________________
| ★ or ☆      |
|---------------|
| New           |
| Read          |
| Replied       |
|---------------|
| User status 1 |
| User status 2 |
| User status 3 |
| User status 4 |
|---------------|
| New status…   |
| Edit status…  |
|---------------|

At the top is the item to star/un-star a mail, depending on its current starry status.
Then come the fixed system statuses.
After that the user statuses.
"New status…" opens a little window with a BTextControl (as it already does in the Mail app) to create a new status text file in ~/config/settings/Mail/status.
"Edit status…" opens ~/config/settings/Mail/status in Tracker so the user can edit/delete a status or create more status directly there.