Split out win related functions to win.c

Also move static function prototypes out of compton.h. Seems like the
previous developers didn't know what header files are for.

Seems to have bugs after the split.

src/x.h

@@ -0,0 +1,37 @@
+#pragma once
+#include <stdbool.h>
+#include <X11/Xlib.h>
+typedef struct session session_t;
+typedef struct winprop winprop_t;
+/**
+ * Get a specific attribute of a window.
+ *
+ * Returns a blank structure if the returned type and format does not
+ * match the requested type and format.
+ *
+ * @param ps current session
+ * @param w window
+ * @param atom atom of attribute to fetch
+ * @param length length to read
+ * @param rtype atom of the requested type
+ * @param rformat requested format
+ * @return a <code>winprop_t</code> structure containing the attribute
+ *    and number of items. A blank one on failure.
+ */
+winprop_t
+wid_get_prop_adv(const session_t *ps, Window w, Atom atom, long offset,
+    long length, Atom rtype, int rformat);
+
+/**
+ * Get the value of a type-<code>Window</code> property of a window.
+ *
+ * @return the value if successful, 0 otherwise
+ */
+Window
+wid_get_prop_window(session_t *ps, Window wid, Atom aprop);
+
+/**
+ * Get the value of a text property of a window.
+ */
+bool wid_get_text_prop(session_t *ps, Window wid, Atom prop,
+    char ***pstrlst, int *pnstr);