editor: Support shift-left/right for selecting text

If the shift modifier is active then we don't make the cursor and the
anchor the same and as a result we develop a selection in the direction
that the arrow key gets pressed in.

https://bugs.freedesktop.org/show_bug.cgi?id=66802

clients/editor.c

@@ -1141,7 +1141,8 @@ key_handler(struct window *window,
 			new_char = utf8_prev_char(entry->text, entry->text + entry->cursor);
 			if (new_char != NULL) {
 				entry->cursor = new_char - entry->text;
-				entry->anchor = entry->cursor;
+				if (!(input_get_modifiers(input) & MOD_SHIFT_MASK))
+					entry->anchor = entry->cursor;
 				widget_schedule_redraw(entry->widget);
 			}
 			break;
@@ -1151,7 +1152,8 @@ key_handler(struct window *window,
 			new_char = utf8_next_char(entry->text + entry->cursor);
 			if (new_char != NULL) {
 				entry->cursor = new_char - entry->text;
-				entry->anchor = entry->cursor;
+				if (!(input_get_modifiers(input) & MOD_SHIFT_MASK))
+					entry->anchor = entry->cursor;
 				widget_schedule_redraw(entry->widget);
 			}
 			break;