Add on-redraw hook command

[joelim-work]

• Fixes Keep leftmost columns at constant width/execute script on window resize? #713

This seems like a nice feature to have, I guess nobody got around to actually implementing this.

tcell.EventResize events will eventually call the redraw command, and I think it might be useful to apply the hook on redraws as opposed to resizes, for instance if the user wants to trigger the hook manually using <c-l>.

Below is an example, which even triggers upon startup:

cmd on-redraw %{{
    if [ $lf_width -le 80 ]; then
        lf -remote "send $id set ratios 1:2"
    elif [ $lf_width -le 160 ]; then
        lf -remote "send $id set ratios 1:2:3"
    else
        lf -remote "send $id set ratios 1:2:3:5"
    fi
}}