Xwt.Gtk: if TextLayout.Trimming is WordElipsis, only the first line is drawn

If TextLayout.Width > 0, Height is -1 and TextLayout.Trimming is WordElipsis, then 
Xwt.Gtk draws a one-liner according to Width, and ellipsize this line:
![texttrimmingellipsisgtkwrong](https://cloud.githubusercontent.com/assets/1479980/3526167/bea71582-0779-11e4-9366-625781c6b53f.png)
Xwt.Wpf makes soft line breaks according to Width, and ellipsize every line that does't fit into Width:
![texttrimmingellipsiswpf](https://cloud.githubusercontent.com/assets/1479980/3526183/e3571eae-0779-11e4-809b-177447a6e84d.png)
if Layout.Height is set, then the last line should be ellipsized to show that there is more content:
![texttrimmingellipsisheigthbreakwpf](https://cloud.githubusercontent.com/assets/1479980/3526208/1b4cbd46-077a-11e4-8eb5-5e5c67d4c3bd.png)
This (Xwt.Wpf) should be the desired behaviour.

The same problem appears with TextLayout.GetSize(): if ellipsized, it returns the Heigth of one line.

This commit produces the following output on Xwt.Gtk:

![texttrimmingellipsisgtk](https://cloud.githubusercontent.com/assets/1479980/3526237/53ce00a8-077a-11e4-8892-a6aabb63c746.png)
![texttrimmingellipsisheigthbreakgtk](https://cloud.githubusercontent.com/assets/1479980/3526303/b5b8444a-077a-11e4-81ed-78c45669dadd.png)