Improve word selection behavior

Improve word selection behavior

* lisp/tab-bar.el (tab-bar-map): Don't bind touch-screen-drag.
* lisp/touch-screen.el (touch-screen-drag): Extend the region
based on the position of the mark, not the position of point
relative to EVENT.
(touch-screen-translate-touch): Don't generate virtual function
keys for non-mouse events.
(function-key-map): Remove redundant definitions.
* src/keyboard.c (read_key_sequence): Don't generate *-bar
prefix keys for mock input (such as input from function key
translation.)