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.

plugins/win-capture/CMakeLists.txt

@@ -1,10 +1,12 @@
 project(win-capture)
 
 set(win-capture_HEADERS
+	window-helpers.h
 	dc-capture.h)
 
 set(win-capture_SOURCES
 	dc-capture.c
+	window-helpers.c
 	monitor-capture.c
 	window-capture.c
 	plugin-main.c)