* keyboard.c (echo_char, read_char): Apply EVENT_HEAD without first

* keyboard.c (echo_char, read_char): Apply EVENT_HEAD without first
	testing for EVENT_HAS_PARAMETERS; EVENT_HEAD works properly on
	all sorts of events now.
	(read_key_sequence): Use the new accessors to decide in which window
	an event occurred.

	* keyboard.c (Qevent_unmodified): Replaced by...
	(Qevent_symbol_elements): New property.
	(syms_of_keyboard): initialize and staticpro the latter, not the
	former.

	* keyboard.c (readable_events): This doesn't need to scan and
	discard mouse release events anymore; it just uses
	EVENT_QUEUES_EMPTY.
	(kbd_buffer_get_event): No need to skip past mouse release events.

	* keyboard.c (button_down_location): New variable, which
	stores the location at which each button was pressed, so we
	can build a complete drag event when the button is released.
	(make_lispy_event): When a button is pressed, record its
	location in button_down_location, and turn it into a `down'
	event.  When a button is released, compare its release
	location with its press location, and decide whether to call
	it a `click' or `drag' event.
	Change mouse movement events to be arranged like click events.
	(format_modifiers): Note that the click modifier has no
	written representation.
	(modifier_names, modifer_symbols): New variables, used to
	create the Qevent_symbol_elements property.
	(modify_event_symbol): Change the format of the modified
	symbol cache; there are too many modifier bits now to use a
	vector indexed by a modifier mask.  Use an assoc-list instead.
	Document the format of the cache.
	Put the Qevent_symbol_elements property on each new symbol,
	instead of a Qevent_unmodified property.
	(symbols_of_keyboard): Put Qevent_symbol_elements properties on
	the symbols specified in head_table, not Qevent_unmodifed properties.
	Initialize and staticpro modifier_symbols, and staticpro the
	window elements of button_down_location.