Add object:state-changed:<STATE> events to event tree

Closes accerciser#34

See merge request !32

Add the "object:state-changed:<STATE>" events for
the specific state changes to the event tree.

This e.g. allows to filter "object:state-changed" events
in Accerciser in a more fine-granular way to only
see those that are actually of interest.

Fixes: accerciser#34

use portable "command -v" to detect installed programs

See merge request !31

The "which" utility is not guaranteed to be installed either, and if it
is, its behavior is not portable either. This means that when various
programs are installed, the `which` check will report a fatal error
because the which tool did not exist and the shell returned a nonzero
status when attempting to fork+exec. If it did exist, it might not be an
implementation of `which` that returns nonzero when commands do not
exist.

The general scripting suggestion is to use the "command -v" shell
builtin; this is required to exist in all POSIX 2008 compliant shells,
and is thus guaranteed to work everywhere.

For some in-depth discussions on the topic, see:
- https://mywiki.wooledge.org/BashFAQ/081
- https://unix.stackexchange.com/questions/85249/why-not-use-which-what-to-use-then/85250#85250

Examples of open-source shells likely to be installed as /bin/sh on
Linux, which implement the 15-year-old standard: ash, bash, busybox,
dash, ksh, mksh and zsh.

Seve...

registry: Remove device_event_register; it's unused

See merge request !28

See at-spi2-core#94 for
details.  This seems to be a vestige of another way of polling for
mouse events, when nowadays it is Registry.registerEventListener() that
is used.

It has been deprecated for a long time. Fixes some compiler warnings. Add a
check for glib >= 2.36.0.

TRACE is not defined anywhere. It's a wonder that this code ever compiled.

Fixes #10

AT-SPI has no corresponding implementation, so the code was unused and
was likely nonfunctional, since it called a function that appears not to have
been defined anywhere.

Helps #10

tests: Include header to get g_marshal_value_peek_boxed definition

See merge request !27

user_marshal.c:55:24: warning: implicit declaration of function
'g_marshal_value_peek_boxed' [-Wimplicit-function-declaration]

Fix attributeListToHash() in case there’s a colon in an attr value

See merge request !26

Make Selection#clearSelection work

See merge request !25

Call the corresponding 'Atspi.Selection' method.

Forwarding to 'Atspi.Text' instead looks like that might
have been a copy-paste error and e.g. resulted in

    AttributeError: type object 'Text' has no attribute 'clear_selection'

when calling

   sel.clearSelection()

on a 'pyatspi.selection.Selection' object in Accerciser's
IPython console.

Similar to the change for getPath, add a safeguard here so that we don't
loop infinitely if we get bad parent data.

Fixes #7

In theory, we should always be able to reach the root accessible by querying
an object's parent, querying that object's parent, and so on. Nevertheless,
we want to be sure we don't hang, so bail out if this fails within a
reasonable number of attempts.

Fixes #6

Add screen-reader:mode-changed signal

See merge request !24

Migrate to gtk3

See merge request !23

Removes the dependency on the deprecated dbus-glib.

Fixes #4