Add slider for volume control

Since the current volume is not known when
the command is sent and executed, the only way to
adjust the volume in a consistent way is to keep
the client window always opened and make the slider
reset its position every time a new media is played.

At the moment the slider is not reset, while in
the final version the media will be started by the
client
Add support for stateful actions. Export Server object.

Reorganise the code to export the Server object rather
than just the CommandSender.

Add the ServerStatus class, which is passed through to
the action received on the server. It can be used inside 
the action object to build the corresponding command
sequence keeping into account any state of the player
on the server side.
Refactor package structure.

Put the OmxPlayerAction class in the 'common' package
and all the derived specific actions into the new
'client.actions' package. The server doesn't need to 
know about specific actions, it just use the generic
superclass.
Fix volume slider. Add 'action completed'
method to OmxPlayerAction.

The volume slider now works as expected.
A new method 'updateServerStatus()' is added
to OmxPlayerAction, which is called when all
the commands specified by the action are
successfully executed on the server.
Add support for response to action requests.

A new interface implemented by all the actions classes
abstracts the action object.

The SendCommandTask now returns a new ActionResponse
object, which contains a (list of) response status.
This status is made of the exit code of the command
executed on the server and of any text output it gave
as a result of its execution.

Once the command is executed on the server, the
SendCommandTask will call the receiveResponse() method
(part of the new Receiver interface) which will handle
the returned response messages.
Add "currentDirectory" to ServerStatus

The new field holds the current directory where 
the server is looking to load media.

The value given in this commit is temporary, it
will be taken from the server configurationfile,
along with some "boudary path" to prevent a remote
user to list the entire filesystem.
Turn Language and Configuration in singleton classes.

Language and configurations must be accessible from
any point in the code, and the information they
provide are always the same.

The Configuration class is now abstract, and the singleton
pattern is actually implemented half in Configuration
(by means of protected constructor and local protected
instance object) and half in the subclasses (by menas
of the getInstance() method).

The language class implements a modified version of the
singleton pattern, which makes sure to be able to
always fetch the text, either from the defaults or from
the language configured in the configuration file, just by
calling the static "get" method. There's no need to first
retreive the actual Language single instance.
Small improvements in the send action/get response mechanism

The updateServerStatus() method is moved up in the Action
interface.

Add getLastMessage() and getLastExitValue() convenience method 
to ActionResponse.

Redirect the standard error to standard output when executing
the requested actions on the server.
Enable the remote start of a media file on the server.

Add the PlayFile action, which tells the server to start
the playback of a selected media file.

The file to be played is chosen through the new
ChooseMediaDialog dialog window, populated with the
reponse message of the ListFiles action.

Add the "current directory" information to the server
status, and use it to decide which folder list when 
the ListFiles action is received.