bulk merge of dev branch

place window number top-left
deferred thumbnail-clone creation

use my special method for determining whether an icon belongs to the current workspace when deciding on icon size

more icon sizing

icon positioning, again

thumbnail positioning

window-number label positioning, once again

only defer drawing the thumbnails behind the icons, not the drawing of the icons

defer thumbnail drawing using idle_add

check for deferred action happening after removal from stage
Merge branch 'dev'
last-minute overview of README
documentation correction (F10->F3)
vertical centering of thumbnail
add GNOME 3.8 keybindings
don't show workspace separators if there are no windows at all to display
make compatible with 1.9 series
add a "Help" item to the applet menu
process new icon visibility on revert of setting
add CHANGELOG.md
minor change to README