input: add tablet pad setup and button handler

Split pad initialization from tablet initialization to
avoid conflicting handler names.
Also reuse 'get_mapped_button'.

src/input/meson.build

@@ -1,6 +1,7 @@
 labwc_sources += files(
   'cursor.c',
   'tablet.c',
+  'tablet_pad.c',
   'gestures.c',
   'input.c',
   'keyboard.c',