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/get_label_text_dimensions.c

@@ -0,0 +1,42 @@
+/*
+ * Determine the dimensions of a piece of text in the standard
+ * font used in GTK interface elements like labels. We do this by
+ * instantiating an actual GtkLabel, and then querying its size.
+ */
+
+#include <gtk/gtk.h>
+#include "putty.h"
+#include "gtkcompat.h"
+#include "gtkmisc.h"
+
+void get_label_text_dimensions(const char *text, int *width, int *height)
+{
+    GtkWidget *label = gtk_label_new(text);
+
+    /*
+     * GTK2 and GTK3 require us to query the size completely
+     * differently. I'm sure there ought to be an easier approach than
+     * the way I'm doing this in GTK3, too!
+     */
+#if GTK_CHECK_VERSION(3,0,0)
+    PangoLayout *layout = gtk_label_get_layout(GTK_LABEL(label));
+    PangoRectangle logrect;
+    pango_layout_get_extents(layout, NULL, &logrect);
+    if (width)
+        *width = logrect.width / PANGO_SCALE;
+    if (height)
+        *height = logrect.height / PANGO_SCALE;
+#else
+    GtkRequisition req;
+    gtk_widget_size_request(label, &req);
+    if (width)
+        *width = req.width;
+    if (height)
+        *height = req.height;
+#endif
+
+    g_object_ref_sink(G_OBJECT(label));
+#if GTK_CHECK_VERSION(2,10,0)
+    g_object_unref(label);
+#endif
+}