Input buttons for ebook

[Mikus Grinbergs]

The "disable keyboard and mouse in ebook mode" discussion in #9180
reminds me that in ebook mode the XO in effect has only 8 "pressable
inputs" (not counting combinations of presses).  That limits the number
of actions the user can command (without going to menus).

Other devices have doubled the number of functions per "pressable input"
by taking into account duration -- a brief PRESS invokes one action, a
prolonged HOLD invokes a different action.  The idea of duration was
advanced 3 years ago (e.g., #2249), but seems to have been dropped.

I sure wish that implementing "what gets done depends upon how long the
button is pressed" (at least for the four buttons on the right) would be
revisited.  That would improve the usability of the XO as ebook reader.

mikus