*Checking for GDK_WINDOWING_WAYLAND breaks the wayland support when built with meson as it is not detected

The g_return*_if_fail() calls should use PREFIX_IS_TYPE_NAME [1], which
returns whether the given instance is of the right type, not
PREFIX_TYPE_NAME [2], which checks whether the given instance if of the
right type, warns if not, and returns the passed-in instance unchanged.

Using the wrong one means the returned value is the passed-in pointer,
and thus passes the g_return*_if_fail() whenever the pointer is
non-NULL, no matter whether it's of the right type or not.

A warning would still be emitted by g_type_check_instance_cast(), but
the execution wouldn't get short-circuited.

[1] calls g_type_check_instance_is_a()
[2] calls g_type_check_instance_cast()

When loaded as in-process applet, we must not call `textdomain()` or we
will override mate-panel's text domain.  Instead, properly use
gi18n-lib.h header that provides the same API but uses the compile-time
`GETTEXT_PACKAGE` value for each call.

Actually drop the whole i18n setup logic that is provided already by
mate-panel's factory macros.

Fixes #203.

* in-process: enable scrolling on icon as in out of process builds

- Use same code to catch scroll events in-process and out