text: Use the widget y coordinate for gestures

text: Use the widget y coordinate for gestures

Currently we use layout coordinates and widget height when determining
where a click or drag has happened. If the widget has top padding (which it
does inside a GtkEntry, for example), the area where it's possible to select
text is shifted down, so the part of GtkText above the layout is not counted
as the draggable area and instead the equal area below the widget is counted.

Since GtkText is always single-line, there's no need to do any of that and
we can use widget coordinates. Then the draggable area matches the widget
and the problems goes away.