Added DisplayText DBUS support

[Mello-Yello]

I added the DisplayText to the Root Interface, because I felt that the Player Interface had more to do about controlling the playback.

To avoid passing the m_osd variable directly to OMXControl, which felt cumbersome, I decided to pass it to OMXPlayerSubtitles. This way OMXPlayerSubtitles.DisplayText() checks whether it's true, and prints the text if that's the case.

I also updated the README and dbuscontrol.sh accordingly.

#587

[torarin]

m_osd has to be initialized to false in the OMXPlayerSubtitles for this to work. However, this is not quite in line with how OMXPlayerSubtitles otherwise works. Most of the functions check m_open to see if Open has been called. Now in DisplayText this is bypassed by the m_osd test. Perhaps you could just make a lambda to send to OMXControl that checks m_osd?

[torarin]

I see now that this a general problem in OMXControl. Calling SetVisible for instance with no subtitles and m_osd = false will trigger the assertion .

[Mello-Yello]

If that's the case would it be better to pass m_osd to OMXControl, so we can also add a check for SetVisible and undo the changes to OMXPlayerSubtitles?

[torarin]

That would work. The most practical thing would probably be to change the asserts in the functions relevant to OMXControl to if (!m_open) return, and remove the NDEBUG guards around m_open.

[Mello-Yello]

Ok, I'll take a look at those assertions later

[torarin]

Looks good. There's also GetActiveStream and SetActiveStream.

[Mello-Yello]

Just one thing: GetActiveStream must return a size_t value.
Would a -1 do it?

Also (I'm new to Git and Github), all this commits will count as one when merged? Otherwise, would it be better If I made another fork so I can make a pull request with just one commit?