widget,widget/material: add selection to the editor

- Allow dragging to be on both horizontal and vertical axes at once.
- Split Editor.caret.pos into caret.start and caret.stop. caret.start is
  the old caret.pos, and is both the position of the caret, and also the
  start of selected text. caret.end is the end of the selected text.
  Start can be after end, e.g. after after Shift-DownArrow.
- Update caret.end after a mouse drag, and various shifted keys
  (Shift-UpArrow, Shift-DownArrow, etc).
- Change Shortcut-C to copy only the selected text, not the whole editor
  text.
- Add Shortcut-X to copy and delete selected text, and Shortcut-A to
  select all text.
- The various Insert/Delete/etc functions now overwrite or delete the
  selection, as appropriate.
- Change MoveCaret to accept a distance for selection end, as well.
  Change SetCaret to accept a selection end offset.
- Add SelectionLen to get the selection length, Selection to get
  selection offsets, SelectedText to get the selected text, and
  ClearSelection to clear the selection.
- Add a rudimentary selection unit test, and extend the deleteWord unit
  test with some text selection cases.
- Add SelectionColor to material.EditorStyle, which defaults to
  Theme.Palette.ContrastBg.

Signed-off-by: Larry Clapp <larry@theclapp.org>

gesture/gesture.go

@@ -90,6 +90,7 @@ type Axis uint8
 const (
 	Horizontal Axis = iota
 	Vertical
+	Both
 )
 
 const (
@@ -201,6 +202,8 @@ func (c *Click) Events(q event.Queue) []ClickEvent {
 	return events
 }
 
+func (ClickEvent) ImplementsEvent() {}
+
 // Add the handler to the operation list to receive scroll events.
 func (s *Scroll) Add(ops *op.Ops) {
 	oph := pointer.InputOp{
@@ -356,6 +359,8 @@ func (d *Drag) Events(cfg unit.Metric, q event.Queue, axis Axis) []pointer.Event
 				e.Position.Y = d.start.Y
 			case Vertical:
 				e.Position.X = d.start.X
+			case Both:
+				// Do nothing
 			}
 			if e.Priority < pointer.Grabbed {
 				diff := e.Position.Sub(d.start)