Dynamic scaling & Graph meter coloring

This patch adds dynamic scaling to the Bar and Graph meter, and color
for Graph meter.

Details of the features:
1. Dynamic scaling is enabled by setting a negative "total" value to
   the meter. It's useful for meter like Load Average and Tasks where
   a fixed maximum value doesn't make sense. For Bar meter a dynamic
   "total" will be computed when drawing. For Graph meter, the "total"
   will align to a power of 2 to ease the management of internal data
   structures.
2. Colors on the graph will base on items of the (stacked) bars.
   The colors are picked to match the percentage of the items as close
   as possible, while the rounding differences (for each character)
   are addressed through the different numbers of braille dots.
   Due to low resolution of the character terminal, the rasterized
   colors may not look nice, but it's better than nothing. :)

[v2: Graph captions now printed above units.
v3: Correction on the "top cell" visuals of the graph, and the
character in no-unicode mode drawing.
v4: GraphData->cell now allocated separately from GraphData.]

Signed-off-by: Kang-Che Sung <explorer09@gmail.com>

Author: Explorer09