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), we get negative Y coordinate inside that padding, and also count an equal area below the GtkText as being inside it.

Since GtkText is always single-line, there's no need to do any of that and we can use widget coordinates. Then using widget height to determine the lower bound makes sense and both problems go away.