dtwm: basic multihead(xinerama only) support

* maximization (also partially fixes strange Firefox behaviour)
* sane initial window placement

Firefox is stranger than most; it appears to maximize to double
X Screen width even on single head displays.  Fixed (on multihead).

cde/programs/dtwm/WmMultiHead.h

@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) 2016 Matthew R. Trower
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to
+ * deal in the Software without restriction, including without limitation the
+ * rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
+ * sell copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+ * IN THE SOFTWARE.
+ */
+
+#ifndef _WmMultiHead_h
+#define _WmMultiHead_h
+
+#include "WmGlobal.h"
+
+typedef struct _WmHeadInfo {
+    int x_org;
+    int y_org;
+    unsigned int width;
+    unsigned int height;
+} WmHeadInfo_t, *WmHeadInfoPtr_t;
+
+WmHeadInfo_t *GetHeadInfo(const ClientData *pcd);
+
+#endif _WmMultiHead_h