Implement touch text selection in GtkEntry

Implement touch text selection in GtkEntry

GtkTextHandle is used to indicate both the cursor position
and the selection bound, dragging the handles will modify
the selection and scroll if necessary.

Backwards text selection is also blocked for touch devices,
so the handles don't get inverted positions (This is more
important though on GtkTextView, as inverted handles may
obscure portions of the selected text, good for consistence
though)