gesturesingle: check for the sequence being handled, not just the event

Checking the return value was valid for most gestures, but
GtkGestureLongPress, where the first press triggers internally an action,
but does nothing for the sequence to be claimed/denied, FALSE was eventually
returned, and the button/sequence functions would be incorrect when
::pressed is emitted.

So check that the sequence is being handled by the gesture, this is more
desirable than the return value as it's independent of sequence state,
and still will be FALSE for the cases we want to catch here.
texthandle: Use the handle widget style context for rendering

Using the parent widget context is a leftover of the pre-popover
implementation, which used GdkWindows directly. This will make the context
reflect widget state, at the expense of changing the selector paths
that used to match the handles.
texthandle: Use active state when handles are being dragged

The active state flags is set on both handles when this happens.
textview: Show magnifier on top of current line

The X coordinate is dependent on touch position, but Y is stepped.
textview: Pass a boolean telling whether a selection drag should extend

Instead of passing a GdkEvent and let the function figure out whether the
selection should be extended, let that to the caller and just pass a
boolean here.
textview: Make "extend selection" only extend

This mode could also shrink the selection, plus the starting point would
seem somewhat arbitrary (actually dependent on the dragging direction of
the last selection).

Made this mode more consistent by only allowing it to extend the selection,
only in one direction for each operation, and so it keeps the current
selection as a minimum.
textview: Implement friendlier touch selection

- It is not possible anymore to trigger text DnD through touch, pressing
  and dragging from within the selection will instead extend it. Text
  shrinking is still available through the handles
- The selection mode for touch is per-word, char-level manipulation is
  still available through the handles.
- Tapping within the selection will bring in text handles, and toggle
  text selection popover.
Adwaita: Make text handles a bit larger

This improves visibility, and interaction, as the invisible rect around is
dependent on the visible handle size
textview: make magnifier size dependent on text size

The magnifier is now set enough height to show the line being currently
manipulated, which makes it just big enough to show the layout height at
that size and magnification.