#266 Migrate PFM to newer framework

[Genreless]

PFM needs to be migrated from .NET 4.0 to a newer framework. This issue was made obvious by attempts to restore functionality to the auto-updater.

[Genreless]

Known options:

1:.NET 4.5 or newer:
    Benefits:
        Same back-end code
    Detriments:
        We lose support for Windows XP
2:Xamarin:
    Benefits:
        We can support macOS and Linux
    Detriments:
        We lose support for Windows XP
3:Mono:
    Benefits:
        We can support macOS and Linux

[daniu]

FYI, there were two reasons I never upgraded to a newer .NET version.

1. to retain backwards compatability for the users - ie not force them to update to keep PFM running. This might be an issue for Mac users especially, I don't think there are a lot of Linux users out there.
2. to be able to do PFM development at the place I worked at the time I was active. There, we had a Linux environment and I could only get Mono running in an older version, ie up to .NET 4.0. Since that is out of the picture, I'd personnally go with option .NET 4.5.

As a side note, the old db table editor support that is still in the code is also a remain of me needing to be able to run PFM on my work machines; WFM wasn't available so the default editor didn't work. Might as well take that code out when updating the framework version.