Speedup overlong text detection

Detecting very long window titles is done via FbTk::TextUtils::doAlignment().
Instead of removing one char from the title at a time to see if it fits into a
given 'max_width', we now use a binary-search like approach to get faster to
the right value. This massively improves the speed for windows with
(arbitrary) long window titles (see bug #1090, javascript

    document.title = new Array(4999).join(".");

leads to massive waiting for fluxbox to detect that this window has a very
long title).

In addition to that Xft returns 'wrapped' shorts ('integer overflows') for
long texts: XGlpyhInfo.xOff is declared as signed short, it's able to hold
~32k pixels. A monospace font with font-size 10 produces an integer
overflow after 3276 chars / glyphs, thus rendering the check

    if (text_width < max_width) { /* ... */ }

pointless and leading rendering the whole title. By calculating some kind of
upper limit for a pseudo-wide glyph ("WW") and strictly cutting off the input
string at that limit prevents this issue.