Tests: move test bodystructures to a common header file

This is done so that we can use the bodystructures in other tests too
Tests: Added unit tests for Imap::Network::MsgPartNetAccessManager
Do not use TreeItems in PartWidgetFactory

The TreeItem class is an implementation detail of the ImapModel and should not
be used outside the ImapModel.

REVIEW: 118228
Gui: hide portWarning, passwordWarning and passwordPluginStatus when not using smtp

REVIEW: 118531