Windows support

[vhakulinen]

At the moment, gnvim doesn't run on windows (anymore). This is because I moved the neovim's subprocess to use GTKs subprocess instead of rust's std Command, so that we do the IO in GTK's main loop and not in separate IO thread and thus avoid some deadlocks.

The problem lies with GTK's stdin/stdout pipes - on windows there is no pollable interface implemented for them (see: https://gitlab.gnome.org/GNOME/glib/-/issues/2016). There are couple of ways to fix this:

• Help with the above glib issue and implement the needed interface
• Incorporate Wing into gnvim (requires FFI crate for said library). This library was mentioned in the glib issue
• Spawn an IO thread with the current implementation when running on windows

Meawhile, if you want to run gnvim on windows, you can use this commit: https://github.com/vhakulinen/gnvim/commit/5339bb566281d21b4f73a68fb54ef2113c77e27e.

[FelipeLema]

do mind that webkit2gtk-4.0 is not being supported in windows for more than a year (see https://github.com/msys2/MINGW-packages/issues/3484#issuecomment-483427316 )