win-capture: Separate window finding source code

Because other capture methods may end up needing to share this code,
separate the window finding source code to window-helpers.c and
window-helpers.h.

This include a function to fill out a property list with windows, a
function to find a window based upon priority/title/class/exe, and a
function to decode the window title/class/exe strings from a window
setting string.
Add debian packaging files