Break up gtkmisc.c.

It's another file that should have been subdivided into lots of tiny
separate things in the utils library - especially since for some
reason I made a completely separate 'guimisc' cmake-level library for
it when there was no need.

unix/utils/string_width.c

@@ -0,0 +1,18 @@
+/*
+ * Return the width of a string in the font used in GTK controls. Used
+ * as a means of picking a sensible size for dialog boxes and pieces
+ * of them, in a way that should adapt sensibly to changes in font and
+ * resolution.
+ */
+
+#include <gtk/gtk.h>
+#include "putty.h"
+#include "gtkcompat.h"
+#include "gtkmisc.h"
+
+int string_width(const char *text)
+{
+    int ret;
+    get_label_text_dimensions(text, &ret, NULL);
+    return ret;
+}