Rework and simplify magit#utils#search_buffer_in_windows usage

This very complicated function has been created for wrong reasons.
vim offers since 7.4.1926 a lot of window related function, such as
'bufwinid()', 'win_gotoid()'...

This function were used in magit#show_magit and magit#jump_to.

magit#show_magit is a simple update.
magit#jump_to goes a bit furhter, to beat existing corner cases.

This new patch implies a check of vim version at startup.