QOL: Window button should focus tabbed windows.

[se5a]

When you click a button to open a window that has been designated to be tabbed with another window,
if the window is in a tab but not focused,
it should *focus* instead of closing,
if it's not in a tab already,
it should add it to the tab *and* focus,
if it's in a tab and focused,
it should close.