drop gtk2 support

When opening pavucontrol.glade with Glade 3.22.1, this message was
shown:

    pavucontrol.glade targets Gtk+ 2.16

    But this version of Glade is for GTK+ 3 only.
    Make sure you can run this project with Glade 3.8 with no deprecated
    widgets first.

I think it's best to just drop the gtk2 support.

configure.ac

@@ -41,26 +41,7 @@ AC_TYPE_SIGNAL
 AC_HEADER_STDC
 AX_CXX_COMPILE_STDCXX_11
 
-AC_ARG_ENABLE(gtk3,
-   [AS_HELP_STRING([--disable-gtk3],
-      [Whether to build or not against gtk3 [default=yes]])],
-   [enable_gtk3=$enableval],
-   [enable_gtk3="yes"])
-
-if test "x$enable_gtk3" = "xyes"; then
-   gtkmm_ver="3.0"
-   gtkmm_min_ver="2.99"
-   canberra_ver="3"
-   AC_DEFINE([HAVE_GTK3], [1], [Define if we are building against gtk3])
-   echo "***"
-   echo "*** Attempting build in GTK3 mode. Use --disable-gtk3 to build for GTK2"
-   echo "***"
-else
-   gtkmm_ver="2.4"
-   gtkmm_min_ver="2.16"
-   canberra_ver=""
-fi
-PKG_CHECK_MODULES(GUILIBS, [ gtkmm-$gtkmm_ver >= $gtkmm_min_ver sigc++-2.0 libcanberra-gtk$canberra_ver >= 0.16 ])
+PKG_CHECK_MODULES(GUILIBS, [ gtkmm-3.0 >= 3.0 sigc++-2.0 libcanberra-gtk3 >= 0.16 ])
 AC_SUBST(GUILIBS_CFLAGS)
 AC_SUBST(GUILIBS_LIBS)
 

src/devicewidget.cc

@@ -70,13 +70,8 @@ DeviceWidget::DeviceWidget(BaseObjectType* cobject, const Glib::RefPtr<Gtk::Buil
     for (unsigned i = 0; i < PA_CHANNELS_MAX; i++)
         channelWidgets[i] = NULL;
 
-#ifdef HAVE_GTK3
     offsetAdjustment = Gtk::Adjustment::create(0.0, -2000.0, 5000.0, 10.0, 50.0, 0.0);
     offsetButton->configure(offsetAdjustment, 0, 2);
-#else
-    offsetAdjustment = new Gtk::Adjustment(0.0, -2000.0, 5000.0, 10.0, 50.0, 0.0);
-    offsetButton->configure(*offsetAdjustment, 0.0, 2);
-#endif  /* HAVE_GTK3 */
 }
 
 void DeviceWidget::init(MainWindow* mainWindow, Glib::ustring deviceType) {

src/devicewidget.h

@@ -101,11 +101,7 @@ protected:
     Gtk::HBox *portSelect, *offsetSelect;
     Gtk::ComboBox *portList;
     Glib::RefPtr<Gtk::ListStore> treeModel;
-#ifdef HAVE_GTK3
     Glib::RefPtr<Gtk::Adjustment> offsetAdjustment;
-#else
-    Gtk::Adjustment *offsetAdjustment;
-#endif /* HAVE_GTK3 */
 
 private:
     Glib::ustring mDeviceType;

src/mainwindow.cc

@@ -196,11 +196,7 @@ MainWindow* MainWindow::create(bool maximize) {
 void MainWindow::on_realize() {
     Gtk::Window::on_realize();
 
-#ifdef HAVE_GTK3
     get_window()->set_cursor(Gdk::Cursor::create(Gdk::WATCH));
-#else
-    get_window()->set_cursor(Gdk::Cursor(Gdk::WATCH));
-#endif /* HAVE_GTK3 */
 }
 
 bool MainWindow::on_key_press_event(GdkEventKey* event) {

src/pavucontrol.glade

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <interface>
-  <requires lib="gtk+" version="2.16"/>
+  <requires lib="gtk+" version="3.0"/>
   <object class="GtkAdjustment" id="adjustment1">
     <property name="upper">100</property>
     <property name="value">44.2408370972</property>