Set the systray plugin free form liblxqt's XfitMan
Relax limits on size and update frequency

The current size and frequency limits appear arbitrary. Some users might want a higher update frequency and/or larger size of the plot. This patch grants them. 100ms is the new lower bound. I understand that lower values might eat up the CPU. The user should realize this latest by looking at the plot itself..
Reset statistics widget only when size changes. Fixes lxde/lxde-qt#353
Panel background configuration

Options to set background color, image, or get from theme