GTK2/3/4: Refactor dw_init(), splitting off path detection into _dw_init_path(). Using argv[0] to detect the executable path is very inaccurate, so we will add platform specific code for Linux and FreeBSD to query the executable path. If platform specific code fails, fall back to argv[0] like before.

# Set these here to change the version for all platforms
DW_MAJOR_VERSION=3
DW_MINOR_VERSION=3