GtkTextView: use GSlice to allocate GtkTextLineSegment's

Use GSlice to allocate all types of segments:
- char
- toggle
- mark
- pixbuf
- child widget

Char segments are a bit more complicated because the length of the text
is determined at run time and stored in the 'byte_count' field. If the
text is long, GSlice will call the system malloc() anyway, so it's
better to always use GSlice for GtkTextLineSegment.

Toggle segments are also freed in gtktextbtree.c, hence the function
_gtk_toggle_segment_free() (for a later commit it would be nice to
rename those functions with the _gtk_text prefix).