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/align_label_left.c

@@ -0,0 +1,20 @@
+/*
+ * Helper function to align the text in a GtkLabel to the left, which
+ * has to be done in several different ways depending on GTK version.
+ */
+
+#include <gtk/gtk.h>
+#include "putty.h"
+#include "gtkcompat.h"
+#include "gtkmisc.h"
+
+void align_label_left(GtkLabel *label)
+{
+#if GTK_CHECK_VERSION(3,16,0)
+    gtk_label_set_xalign(label, 0.0);
+#elif GTK_CHECK_VERSION(3,14,0)
+    gtk_widget_set_halign(GTK_WIDGET(label), GTK_ALIGN_START);
+#else
+    gtk_misc_set_alignment(GTK_MISC(label), 0.0, 0.0);
+#endif
+}