Use Config.PACKAGE_NAME instead of "LaTeXila"

For the window title, remove the code because the title is handled by
Tepl.

src/latexila_app.vala

@@ -239,7 +239,7 @@ public class LatexilaApp : Gtk.Application
         }
 
         Gtk.show_about_dialog (get_active_main_window (),
-            "program-name", "LaTeXila",
+            "program-name", Config.PACKAGE_NAME,
             "version", Config.PACKAGE_VERSION,
             "authors", authors,
             "artists", artists,

src/main_window.vala

@@ -152,7 +152,6 @@ public class MainWindow : ApplicationWindow
     public MainWindow (LatexilaApp app)
     {
         Object (application: app);
-        this.title = "LaTeXila";
 
         // Init TeplApplicationWindow GActions.
         Tepl.ApplicationWindow tepl_window =