Use floating-point instead of fixed-point CPU load values

This patch removes CPU_SCALE=256 in favor of storing CPU load and history values as 32-bit floats. A small benefit of this change is that this removes truncation inaccuracies resulting from the conversion of CPU load values usually sampled in /proc/stat with resolution of 100 Hz into the now-removed fixed-point resolution of 256 (8 bits).

The CPU history graph draw area is now displaying the last point (or last line, depending on display mode) using alpha-blending.

The thickness of the CPU usage line in grid mode has been increased from 1.0 to 1.75 to make the line appear to be above the grid rather than below the grid.