[PATCH] plugins/tasklist can sort window buttons by monitor

Hello!

I added new feature to plugins/tasklist of panel. Let me explain please why and what it does.

My normal setup is 2 monitors and only one panel at one of monitors. So second (main) monitor is completely clear from panels, widgets, etc.
And on my panel I would like to have two tasklists which would show window buttons for specific monitor only.
Here is an example (with two tasklist plugins running):

| /app1/app2/app3/ | CLOCK | /app4/app5/app6/ |

Where app1, app2, app3 are on Monitor-1 and app4, app5, app6 are on Monitor-2.

I would say I updated "Show windows from all monitors" feature.

xfce-4.18.patch This patch uses xfce-4.18 branch as source branch.