add Window::display_selection_at(LineCount)
add z prefixed 'window commands'

* zz or zc for centering selection on window (replaces alt-c)
* zt and zb for placing the main selection on top/bottom
update README
ctags.kak: <a-c> is now zc
git-tools: fix git-diff-show
delete buffer if needed when creating a scratch buffer
git-tools.kak: initial git-show command
hide ncurses.h include in ncurses.cc
Add zj and zk command for scrolling the window