gdk: Add GdkDevicePad

This is an interface meant to be implemented by the "pad" devices.
This device-specific interface exposes the mapping of all pad features,
it allows retrieving:
- The number of buttons/rings/strips
- The number of groups
- The number of modes a group has
- Whether a given button/ring/strip belongs to a given group

https://bugzilla.gnome.org/show_bug.cgi?id=770026

docs/reference/gdk/gdk-docs.sgml

@@ -25,6 +25,7 @@
     <xi:include href="xml/gdkseat.xml" />
     <xi:include href="xml/gdkmonitor.xml" />
     <xi:include href="xml/gdkdevice.xml" />
+    <xi:include href="xml/gdkdevicepad.xml" />
     <xi:include href="xml/regions.xml" />
     <xi:include href="xml/pixbufs.xml" />
     <xi:include href="xml/rgba_colors.xml" />

docs/reference/gdk/gdk3-sections.txt

@@ -796,6 +796,25 @@ gdk_device_type_get_type
 GDK_MAX_TIMECOORD_AXES
 </SECTION>
 
+<SECTION>
+<TITLE>GdkDevicePad</TITLE>
+<FILE>gdkdevicepad</FILE>
+GdkDevicePad
+GdkDevicePadFeature
+gdk_device_pad_get_n_groups
+gdk_device_pad_get_group_n_modes
+gdk_device_pad_get_n_features
+gdk_device_pad_get_feature_group
+
+<SUBSECTION Standard>
+GDK_TYPE_DEVICE_PAD
+GDK_DEVICE_PAD
+GDK_IS_DEVICE_PAD
+
+<SUBSECTION Private>
+gdk_device_pad_get_type
+</SECTION>
+
 <SECTION>
 <TITLE>GdkSeat</TITLE>
 <FILE>gdkseat</FILE>

docs/reference/gdk/gdk3.types

@@ -4,6 +4,7 @@ gdk_app_launch_context_get_type
 gdk_cursor_get_type
 gdk_device_get_type
 gdk_device_manager_get_type
+gdk_device_pad_get_type
 gdk_display_get_type
 gdk_display_manager_get_type
 gdk_drag_context_get_type