Clean up signal.h a little
Use unique_ptr in Connection rather than a shared_ptr
Redesign VideoFrame

Eliminate the manual memory management and shuffle around where the
copies are made to eliminate the need for non-owning video frames.
Fix the Select None button in the export dialog
Use gil in ColourButton rather than wxMemoryDC

Basic drawing with wxMemoryDC is relatively slow.
Slightly simplify spectrum image generation in the colorpicker
Bump dependency versions
Retry commits of file writes for up to a second to work around AV scanning

Poorly-written antivirus software briefly lock newly written files to
scan them for viruses, which makes the rename from the temp file to
actual file fail. Work around this by retrying the rename up to ten
times.

Closes #1620.
Eliminate double dot in temporary file names

Probably didn't cause any problems, but it was slightly ugly.