Nextstep: Use instancetype explicit return type

Nextstep: Use instancetype explicit return type

This removes compiler warnings on macOS and improves type safety.

* nsterm.m (initFrameFromEmacs:):
(menuDown:):
(toolbarClicked:):
(toggleToolbar:):
(setMiniwindowImage:):
(initFrame:window:):
(condemn, reprieve, setPosition:portion:whole:):
(repeatScroll:):
* nsmenu.m (initWithTitle:):
(initWithTitle:frame:):
(initForView:withIdentifier:):
(init, initWithContentRect:styleMask:backing:defer:):
(initFromContents:isQuestion:):
* nsimage.m (allocInitFromFile:):
(initFromXBM:width:height:fg:bg:):
(setXBMColor:):
(initForXPMWithDepth:width:height:): Use instancetype as return
type instead of implicit id.