Add support for per-core history graphs

This patch enables the plugin to display the histories of all CPU cores next to each other.

The amount of spacing between the history graphs currently depends on the "Show border" setting: if "Show border" is enabled then the spacing between cores is 1 pixel, otherwise it is zero.

For core counts higher than 4, this can make the plugin take a lot of space in the panel (depending on the size of displayed history).

The implementation internally stores the histories of all CPU cores irrespective of whether "Per-core history graphs" setting is enabled or disabled. This has almost no negative performance impact, but it can increase the plugin's memory consumption on many-core CPUs if the history size is large.

Closes: #15 (closed)