Fix some build warnings

[ryonakano]

After

user@VirtualBox-36929c8d:~/work/switchboard-plug-wacom$ ninja -C builddir/ install
ninja: Entering directory `builddir/'
[1/15] Compiling Vala source ../src/Backend/Device.vala ../src/Backend/DeviceManager.vala ../src/Backend/DeviceManage...w.vala ../src/Views/TabletView.vala ../src/Widgets/DrawingArea.vala ../src/MainPage.vala ../src/Plug.vala Config.vala
../src/Views/StylusView.vala:107.32-107.47: warning: `Gtk.ComboBoxText' has been deprecated since 4.10
  107 |         var button_combo = new Gtk.ComboBoxText () {
      |                                ^~~~~~~~~~~~~~~~     
../src/Views/StylusView.vala:107.13-107.24: warning: `Gtk.ComboBoxText' has been deprecated since 4.10
  107 |         var button_combo = new Gtk.ComboBoxText () {
      |             ^~~~~~~~~~~~                            
../src/Views/TabletView.vala:43.5-43.48: warning: `Gtk.ComboBoxText' has been deprecated since 4.10
   43 |     private Gtk.ComboBoxText tracking_mode_combo;
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
../src/Views/TabletView.vala:50.35-50.50: warning: `Gtk.ComboBoxText' has been deprecated since 4.10
   50 |         tracking_mode_combo = new Gtk.ComboBoxText ();
      |                                   ^~~~~~~~~~~~~~~~    
Compilation succeeded - 4 warning(s)

Before

Latest main (06e3765)

user@VirtualBox-36929c8d:~/work/switchboard-plug-wacom$ ninja -C builddir/ install
ninja: Entering directory `builddir/'
[1/15] Compiling Vala source ../src/Backend/Device.vala ../src/Backend/DeviceManager.vala ../src/Backend/DeviceManage...w.vala ../src/Views/TabletView.vala ../src/Widgets/DrawingArea.vala ../src/MainPage.vala ../src/Plug.vala Config.vala
../src/Backend/DeviceManagerWayland.vala:100.17-100.32: note: Assignment: Unsafe conversion from `int' to `Wacom.Backend.Device.DeviceType'
  100 |                 type |= (1 << i);
      |                 ^~~~~~~~~~~~~~~~ 
../src/Backend/DeviceManagerWayland.vala:152.25-152.68: warning: Access to possible `null'. Perform a check or use an unsafe cast.
  152 |         var node_path = (device as Gdk.Wayland.Device).get_node_path ();
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~    
../src/Backend/WacomToolMap.vala:247.21-247.88: warning: unhandled error `WacomException'
  247 |                     throw new WacomException.LIBWACOM_ERROR (error.get_message () ?? "");
      |                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
../src/Views/StylusView.vala:107.32-107.47: warning: `Gtk.ComboBoxText' has been deprecated since 4.10
  107 |         var button_combo = new Gtk.ComboBoxText () {
      |                                ^~~~~~~~~~~~~~~~     
../src/Views/StylusView.vala:107.13-107.24: warning: `Gtk.ComboBoxText' has been deprecated since 4.10
  107 |         var button_combo = new Gtk.ComboBoxText () {
      |             ^~~~~~~~~~~~                            
../src/Views/TabletView.vala:43.5-43.48: warning: `Gtk.ComboBoxText' has been deprecated since 4.10
   43 |     private Gtk.ComboBoxText tracking_mode_combo;
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
../src/Views/TabletView.vala:50.35-50.50: warning: `Gtk.ComboBoxText' has been deprecated since 4.10
   50 |         tracking_mode_combo = new Gtk.ComboBoxText ();
      |                                   ^~~~~~~~~~~~~~~~    
Compilation succeeded - 6 warning(s)