Add support for exporting to various Megavoice models

[tombogle]

See https://jira.sil.org/browse/HT-69 and https://docs.google.com/document/d/1X3V0JlEjrGBXde7l3crq34F15DC1E1IHuhIh-3TuOaA/edit#heading=h.wwu6ti818u8l
Note: I could not find a definitive set of rules for each Megavioice model, so I sent a request to ask them how to find that info.

[tombogle]

If you install the SaberCopy utlity app from GRN, you can find the specific folder/file rules for each device by creating a new project for the device, and then selecting the "Target device" tab. Some analysis will be needed to see whether all the devices fall into 2 or 3 categories (as far as HearThis is concerned) or whether we actually need to provide a long list of devices and let the user select a specific model. If the former, then we can probably just add another radio button or two (with a tool tip or help link to provide details). If the former, we'll probably need a drop-down box or separate dialog box to select the model. Either way, we'll need to ensure that the existing Megavoice setting translates to the most popular/common model that is compatible with the current behavior of HearThis and that the settings then correctly store the users' choice and restore the selections when they return to the export dialog box.