[WIP] Add application grouping to taskbar

Fixes lxde/lxqt#132.
About the points 1, 2 - You are right
but I don't understand the point 3.  
How does this feature work - you click on a group, popup shows, you hover on another group and popup is switched according to the another group.
"Mouse hover switches visible group"  feature could be enabled only when "Mouse over shows popup" is not checked. When the "Mouse over shows popup" then logic doesn't care about check state of "Mouse hover switches visible group" and the checkbox becomes disabled and state of checkbox is not changed.