Remove project int types: Case short_u: Replace with uint16_t.

- Replace short_u with uint16_t (same size, should give no problems).
- When possible, remove explicit downcasts so that they are found when
  flag -Wconversion enabled.
- Remove short_u typedef.

Requested in #459.
Remove project int types: Case int_u: Replace with uint32_t.

- Replace int_u with uint32_t (same size, should give no problems).
  In fact, only usage found seems to be dead code (some functions in
  ui.h seem not to be used anymore).
- Remove int_u typedef.

Requested in #459.
Remove project int types: Case long_i: Replace with plain long.

Replace long_i with plain long.

long_i was just plain long, adding marker __w64, to be used by
Microsoft's compilers only, as an aid when transitioning from 32 bits to
64 bits. Purpose of this marker was, in fact, to make a 32 bit compiler
emit the same warnings that a 64 bit compiler would.

This __w64 marker is nowadays deprecated by said compilers, and use of a
real 64 bit compiler is recommended instead. See
http://msdn.microsoft.com/en-us/library/s04b5w00.aspx for details.

So, there's no reason to maintain this anymore, and thus is removed.

Refactoring long into int64_t is not attempted, as doing that in a bulk
way is too much complicated. That is left to be done later, on a
file-by-file basis, probably intermixed with file-by-file -Wconversion
activation.

Requested in #459.