Add a thumbnailer for Windows .exe files

[probonopd]

/usr/local/share/thumbnailers/exe-thumbnailer.thumbnailer

[Thumbnailer Entry]
Exec=exe-thumbnailer -v -s %s %i %o
MimeType=application/x-ms-dos-executable;application/x-dosexec;application/x-msdownload

Needs sudo python3 -m pip install icoextract as this is not in FreeBSD Ports.

[probonopd]

Alternatively, we may use something like https://github.com/azsn/get-exe-icon to do this natively in Filer.

[probonopd]

Looking at how Windows stores icons in PE files, it may even be possible to do this natively in C++/Qt5 without the use of additional dependencies.