Improve robustness xterm event processing

Improve robustness xterm event processing

We used to treat the start of a focus-in, focus-out, and the start of
a paste sequence as normal events bound in global-map, but this
approach produces problems when we recognize events in the middle of
actions that don't immediately dispatch to the command loop.

Now we handle these events internally inside read-key, translating the
focus events to nothing and paste-start into an xterm-paste event that
neatly encapsulates the entire paste sequence.

* lisp/term/xterm.el:
(xterm-paste): Accept an event argument; insert text from event.
(xterm-translate-focus-in,xterm-translate-focus-out)
(xterm-translate-bracketed-paste): New functions.
(xterm-handle-focus-in,xterm-handle-focus-out): Remove.
(xterm-rxvt-function-map): Bind new translation functions.