Updated the ZX Spectrum emulator - Fuse.

- Add SDL 2 compatibility
- Make 2x the default display scaler so Fuse is more usable on modern screens
- Speed up display dirty-bit scanning for improved rendering performance
- Debugger: remove $-prefixed hex syntax to avoid clashes with variables, and fix missing bit numbers in DD/FD CB disassembly
- GTK UI: fix loading binary data from deleted files and avoid redraw seams in GTK3
- Unix: store the configuration file in the XDG configuration directory, while still reading ~/.fuserc as a fallback
- Win32 UI: if networking is blocked at startup, disable Spectranet with a warning instead of aborting
- Remove GTK2 support from the build and codebase
- Add and expand unit tests, including display rendering, dirty-region handling and disassembler regression coverage
- Various minor bugfixes and cleanups