Added and used text_style_* methods.

Added and used text_style_* methods.

It had the following functions:
- text_style_New  -- allocates default text style
- text_style_Dup  -- allocates and copies text style
- text_style_Copy -- copies text style
- text_style_Delete -- deallocates text style

NOTE that the vout_ShowText* functions will take a p_style and copy
it internally so you have to free in the caller function.
 Other modules where p_style was malloced there are now fixed to
copy before calling the vout* functions.

Original patch by basOS G with a few modifications by fenrir.

Signed-off-by: Laurent Aimar <fenrir@videolan.org>