Upload Engines & GUI Build Rewrite

[Decimation]

See #80 for context

Unfortunately it seems that every upload engine available in the GUI version is either no longer available or changed its API.

The GUI version hasn't been actively developed or updated, as the cross-platform Rdx version has taken priority. Over time, this has come with significant refactors and changes to the underlying codebase, upon which both the Rdx and GUI version are built. The GUI version has not been updated for these changes (dfdb8d9 is the last commit), and doing so is tantamount to a full rewrite.

Therefore, the current (WPF) GUI version is now deprecated, and an (Avalonia) GUI is in progress but lower priority than Rdx.