core implementation of TabGroup, stubs
basic tab layout
tab header skin
render tab headers
draw tab body borders and background
keep our own reference so we can switch tabs out without them being freed
switch tabs on click
move tab selection and tab-at-point code out to separate methods
tab hover effect
tab header font
support arbitrary widgets in tab headers
initial support for collapsing the tabs
collapse the layout as well
make some space in the skin
separate header skin for collapsed