Bugzilla

After we introduced a `MailFolderId`, this patch allows using the `MailFolderId` in addition to the `MailFolder` object in all folder related functions in MV2.

Following a discussion in the last add-on developer call, where the following was raised, I will change the `MailFolderId` to be an integer in one of the next patches. The reason for this is:
 - the current API allows using an arbitrary path, and even though the docs say to NOT create a path manually, add-on developers do that, which causes	bugs and headaches.
 - the current `MailFolderId` has the same flaw
 - an integer id will make the API more robust
 - I also hope that I can make the integer id follow folder moves.
 
**As a consequence, this patch enforces the usage of the `MailFolderId` in MV3, by deprecating the `MailFolder` object as input parameter.**
 
There is a loose end which will be tackled in the next patch: Some functions (like the copy/create/move folder function) allow specifying an account to access the root folder. This will be solved by introducing a real root folder, which will have a `MailFolderId` and can be used like any other folder, and we no longer need to use accounts as "destination" for certain folder operations in MV3.

I updated all except 3 tests to use `MailFolderId`s, so we can test both supported variations in MV2.

In all tests which have been adjusted, I also minimized creating `MailFolder` objects or `MailFolderId`s manually