[dev] [st] revised mouse handling/reporting

Hi,

the attached patch resolves some minor issues of mouse handling and
changes some of its behaviours:

* Fixed: double-clicking on first word of a line does not select it

* Text is only selected with Button1; pasting when Button2 is released
  (maybe in the future: Button3 to expand the current selection like in
  xterm/rxvt-unicode)

* Support for (xterm-specific) '\E[1002' button-event-tracking: report
  mouse motion events while some button is pressed (only if pointer is
  moved to another character; used in e.g. `vim -c "set mouse=a
  ttymouse=xterm2"'; the old '\E[1000' is kept working)

* When MODE_MOUSE is set: only report mouse events but do not handle
  them inside st, selecting text in vim was kind of awkward, because it
  was selected inside and outside of vim and caused double (i.e. no)
  highlighting of the selected text

* Listen for MotionNotify events only when at least one button is down
  (PointerMotionMask -> ButtonMotionMask)

Regards, Bert