Add BarGraph widget & system usage showcase

[a-dmg]

Description

• introduce a new widget bargraph, it uses dotted bar to show the evolution of a value over time.
• add freq_avg to EWW_CPU builtin constant
• add EWW_HOSTNAME constant

Usage

CPU frequency

(defwidget cpu-usage []
	(box :class "widget-box" :orientation "v" :space-evenly false
		(box :class ""
			(label :halign "start"
				:limit-width 25
				:text "Cpu")
			(label :halign "end"
				:text "${round(EWW_CPU.avg, 2)} ${round((EWW_CPU.freq_avg/1000), 2)}Ghz"))
		(bargraph
			:hexpand true :vexpand true 
			:gradiant-style "fire"
			:value {EWW_CPU.avg}
			:min 0.0
			:max 100.0
			:time-range 150000
			:radius 2
			:dynamic false)
))

Network usage

(defwidget net-usage []
	(box :class "widget-box" :orientation "v" :space-evenly false
		(box :class ""
			(eventbox
				:onclick "eww update IFACE_IDX=${IFACE_IDX + 1 == arraylength(IFACE) ? 0 : IFACE_IDX + 1}"
				:onrightclick "eww update IFACE_IDX=${IFACE_IDX == 0 ? arraylength(IFACE)-1 : IFACE_IDX - 1}"
				(label :class ""
					:halign "start"
					:text "  ${IFACE[IFACE_IDX]}"))
			(label :halign "center"
				:text "Upload ${formatbytes(ceil(EWW_NET["${IFACE[IFACE_IDX]}"].NET_UP))}")
			(label :class ""
				:halign "end"
				:text "Download  ${formatbytes(ceil(EWW_NET["${IFACE[IFACE_IDX]}"].NET_DOWN))}"))
		(box :orientation "v" :hexpand true :vexpand true 
			(bargraph
				:hexpand true :vexpand true 
				:value {EWW_NET["${IFACE[IFACE_IDX]}"].NET_DOWN}
				:gradiant-style "dracula"
				:time-range 150000
				:radius 2
				:dynamic true)
			(bargraph
				:hexpand true :vexpand true 
				:value {EWW_NET["${IFACE[IFACE_IDX]}"].NET_UP}
				:flip-y false
				:gradiant-style "dracula"
				:time-range 150000
				:radius 2
				:dynamic true))
))

Showcase

System usage in a btop like fashion:

Additional Notes

• The implementation is based on the existing graph widget
• The spacing between dots is configurable via time-range and radius which could be confusing at first...

Checklist

Please make sure you can check all the boxes that apply to this PR.

• All widgets I've added are correctly documented. (I hope so...)
• I added my changes to CHANGELOG.md, if appropriate.
• The documentation in the docs/content/main directory has been adjusted to reflect my changes. (not sure if I need to)
• I used cargo fmt to automatically format all code before committing

[coderPaddyS]

I've tried this bar-graph feature and it is a) working as expected and b) a nice addition to my bar adding a cpu utilization history. It would be nice if this is added in the near future.

[gwenhael-le-moine]

The screenshots look good.

One nitpick I have is what does adding EWW_HOSTNAME have to do with this? Especially when we can already use get_env("HOSTNAME")