patch 8.0.1668: terminal debugger: can't re-open source code window

Problem:    Terminal debugger: can't re-open source code window.
Solution:   Add the :Source command.  Also create the window if needed when
            gdb stops at a source line.

runtime/doc/terminal.txt

@@ -731,6 +731,11 @@ to have the 'mouse' option set to enable mouse clicks.
You can add the window toolbar in other windows you open with: >
  :Winbar

If gdb stops at a source line and there is no window currently showing the
source code, a new window will be created for the source code.  This also
happens if the buffer in the source code window has been modified and can't be
abandoned.


Inspecting variables ~
							*termdebug-variables*
@@ -745,8 +750,10 @@ You can usually shorten `:Evaluate` to `:Ev`.

Other commands ~
							*termdebug-commands*
 :Gdb	       jump to the gdb window
 :Program      jump to the window with the running program
 :Gdb	     jump to the gdb window
 :Program    jump to the window with the running program
 :Source     jump to the window with the source code, create it if there
	     isn't one


Communication ~

runtime/pack/dist/opt/termdebug/plugin/termdebug.vim

@@ -246,6 +246,7 @@ func s:InstallCommands()
  command -range -nargs=* Evaluate call s:Evaluate(<range>, <q-args>)
  command Gdb call win_gotoid(s:gdbwin)
  command Program call win_gotoid(s:ptywin)
  command Source call s:GotoStartwinOrCreateIt()
  command Winbar call s:InstallWinbar()

  " TODO: can the K mapping be restored?
@@ -269,13 +270,15 @@ let s:winbar_winids = []

" Install the window toolbar in the current window.
func s:InstallWinbar()
  nnoremenu WinBar.Step   :Step<CR>
  nnoremenu WinBar.Next   :Over<CR>
  nnoremenu WinBar.Finish :Finish<CR>
  nnoremenu WinBar.Cont   :Continue<CR>
  nnoremenu WinBar.Stop   :Stop<CR>
  nnoremenu WinBar.Eval   :Evaluate<CR>
  call add(s:winbar_winids, win_getid(winnr()))
  if has('menu') && &mouse != ''
    nnoremenu WinBar.Step   :Step<CR>
    nnoremenu WinBar.Next   :Over<CR>
    nnoremenu WinBar.Finish :Finish<CR>
    nnoremenu WinBar.Cont   :Continue<CR>
    nnoremenu WinBar.Stop   :Stop<CR>
    nnoremenu WinBar.Eval   :Evaluate<CR>
    call add(s:winbar_winids, win_getid(winnr()))
  endif
endfunc

" Delete installed debugger commands in the current window.
@@ -450,6 +453,14 @@ func s:HandleError(msg)
  echoerr substitute(a:msg, '.*msg="\(.*\)"', '\1', '')
endfunc

func s:GotoStartwinOrCreateIt()
  if !win_gotoid(s:startwin)
    new
    let s:startwin = win_getid(winnr())
    call s:InstallWinbar()
  endif
endfunc

" Handle stopping and running message from gdb.
" Will update the sign that shows the current position.
func s:HandleCursor(msg)
@@ -461,31 +472,32 @@ func s:HandleCursor(msg)
    let s:stopped = 0
  endif

  if win_gotoid(s:startwin)
    let fname = substitute(a:msg, '.*fullname="\([^"]*\)".*', '\1', '')
    if a:msg =~ '^\(\*stopped\|=thread-selected\)' && filereadable(fname)
      let lnum = substitute(a:msg, '.*line="\([^"]*\)".*', '\1', '')
      if lnum =~ '^[0-9]*$'
	if expand('%:p') != fnamemodify(fname, ':p')
	  if &modified
	    " TODO: find existing window
	    exe 'split ' . fnameescape(fname)
	    let s:startwin = win_getid(winnr())
	  else
	    exe 'edit ' . fnameescape(fname)
	  endif
  call s:GotoStartwinOrCreateIt()

  let fname = substitute(a:msg, '.*fullname="\([^"]*\)".*', '\1', '')
  if a:msg =~ '^\(\*stopped\|=thread-selected\)' && filereadable(fname)
    let lnum = substitute(a:msg, '.*line="\([^"]*\)".*', '\1', '')
    if lnum =~ '^[0-9]*$'
      if expand('%:p') != fnamemodify(fname, ':p')
	if &modified
	  " TODO: find existing window
	  exe 'split ' . fnameescape(fname)
	  let s:startwin = win_getid(winnr())
	  call s:InstallWinbar()
	else
	  exe 'edit ' . fnameescape(fname)
	endif
	exe lnum
	exe 'sign unplace ' . s:pc_id
	exe 'sign place ' . s:pc_id . ' line=' . lnum . ' name=debugPC file=' . fname
	setlocal signcolumn=yes
      endif
    else
      exe lnum
      exe 'sign unplace ' . s:pc_id
      exe 'sign place ' . s:pc_id . ' line=' . lnum . ' name=debugPC file=' . fname
      setlocal signcolumn=yes
    endif

    call win_gotoid(wid)
  else
    exe 'sign unplace ' . s:pc_id
  endif

  call win_gotoid(wid)
endfunc

" Handle setting a breakpoint

src/version.c

@@ -762,6 +762,8 @@ static char *(features[]) =

static int included_patches[] =
{   /* Add new patch number below this line */
/**/
    1668,
/**/
    1667,
/**/