[WIP] Add application grouping to taskbar

Fixes lxde/lxqt#132.
But I don't understand why is the code here. In master branch - Icon size of taskbuttons is resized automatically from clouds (probably plugin system of panel do it for me), I tried to comment out this code and icon size of taskbutton is adjusted correctly when I change icon size in panel config dialog.

And second question - Is there any particular reason to change icon geometry through NETWM? we can just change icon size in QToolButton.