port-to-gtk3: Rework Gtk header includes (#7)

In Gtk3 individual headers cannot be included directly in application
code, only the header <gtk/gtk.h> can be included (with a few exceptions
for some platform specific headers).

This has always been considered good practice even for Gtk2, though was
not a hard requirement.

In Gtk3 this is enforced by preprocessor checks.  Failure to do so
yields a preprocessor error and compilation fails:

  "error: Only <gtk/gtk.h> can be included directly."

Change specific Gtk header includes to:

  #include <gtk/gtk.h>

References:

[1] Migrating from GTK+ 2.x to GTK+ 3 - "Do not include individual headers"
    https://developer.gnome.org/gtk3/stable/gtk-migrating-2-to-3.html#id-1.6.3.3.3

[2] Commit - "Remove all traces of GDK_PIXBUF/GTK_DISABLE_SINGLE_INCLUDES"
    5e29973773

Closes #7 - Port to Gtk3

src/DialogPasswordEntry.cc

@@ -21,7 +21,8 @@
 #include <gtkmm/box.h>
 #include <gtkmm/stock.h>
 #include <gtkmm/button.h>
-#include <gtk/gtkentry.h>
+#include <gtk/gtk.h>
+
 
 namespace GParted
 {