Enable users to export conversation as a text file

[pospeselr]

This will require a couple pieces of work:

• UI to initiated an export; this should probably mirror the Send File... command in the context menu when right-clicking on a contact
• We will want to export chats with timestamps, along with relevant events (login, logoff, file transfer initiation/acceptance/cancelling/errors/etc)
• Login/loggoff events would requrie a bit of refactoring of ConversationModel to record login/loggoff events, and could come in in a later patch (or we can ignore these :shrug: )

[m-simonelli]

Resolved in #93