Fix bug in opening file whose names contains spaces

The code to open files whose names contain spaces had an off-by-1
error and didn't work when right-clicking a selection including
quotes. Fix this.

expandfile.go

@@ -44,9 +44,11 @@ func findquotedcontext(t *Text, q0 int) (qq0, qq1 int) {
 	}
 	// TODO(rjk): Should give up at a max filename length.
 	for qq1 < t.file.Nr() {
-		c := t.ReadC(qq1 - 1)
+		// TODO(rjk): why -1?
+		c := t.ReadC(qq1)
+		// c := t.ReadC(qq1 - 1)
 		if c == '\'' {
-			break
+			return qq0, qq1 + 1
 		}
 		if !isfilec(c) && !isfilespace(c) {
 			return q0, q0 // No quote found rightwards.

file/observable_editable_buffer.go

@@ -463,6 +463,10 @@ func (e *ObservableEditableBuffer) String() string {
 	return e.f.String()
 }
 
+func (e *ObservableEditableBuffer) StringSlice(rq0 int, rq1 int) string {
+	return e.f.StringSlice(rq0, rq1)
+}
+
 // ResetBuffer is a forwarding function for rune_array.Reset. Equivalent
 // to re-creating the buffer.
 func (e *ObservableEditableBuffer) ResetBuffer() {