Add ctrl-o to show the entry without closing fzf (#1621)

Files, GFiles, GFiles?, Rg/Ag, Buffers, History, Locate, Lines, BLines,
Tags, BTags, Marks, Changes, Commits and BCommits. Writes the entry to an
fzf#vim#ipc fifo, so it needs the asynchronous fzf#run from
junegunn/fzf@5cb7bab7 (junegunn/fzf#4897, 0.74.4), which the g:loaded_fzf
check detects.

Commands declare '_show' for the entry kind and '_hint' for footer keys, so
the footer and the binding cannot disagree. s:can_show withholds both when
the run would block, when the window to open in is not visible, or when
g:fzf_action, the paste key or the command itself already claims the key.

g:fzf_vim.show_key configures the key, empty to turn it off, comma-separated
to bind several ('ctrl-o,double-click'). Named for its 'Show' footer label,
since Enter already occupies 'Open' there. Only the first key is labelled,
and a clash on any of them drops all of them, so the footer cannot list a
key that does nothing. Binding Enter drops its 'Open' hint too, since fzf
accepts on Enter rather than through g:fzf_action.

s:show_entry parses each entry the way that command's sink does and runs
win_execute() in the window fzf started from. That keeps the focus in fzf
and works from a popup, which win_gotoid() cannot leave. It opens with
'keepalt keepjumps hide', so stepping through entries leaves the alternate
file and the jumplist unchanged. BLines, BTags and a lowercase mark carry a
position but no buffer, so the callback carries the buffer the run started
on as well as the window.

fzf#run is asynchronous, so runs can overlap, and a second run would stop
the first's ipc channel and overwrite the script locals its sink reads back
afterwards. Channels are now keyed by fifo path and per-run state is bound
into the callback instead, which is why Buffers, Colors, Jumps, Maps,
Helptags and complete change here.

README.md

@@ -102,6 +102,15 @@ Commands
   line content (`:Lines`, `:BLines`, `:Marks`, `:Changes`), matched text
   (`:Rg`, `:Ag`, `:Grep`), tag name (`:Tags`, `:BTags`), or commit hash
   (`:Commits`, `:BCommits`)
+- Most commands support `CTRL-O` to open the current entry in the window fzf
+  was started from, leaving fzf open, so you can look through several entries
+  in a single run (`:Files`, `:GFiles`, `:GFiles?`, `:Buffers`, `:History`,
+  `:Locate`, `:Rg`, `:Ag`, `:Grep`, `:Lines`, `:BLines`, `:Tags`, `:BTags`,
+  `:Marks`, `:Changes`, `:Commits`, `:BCommits`). Requires fzf 0.74.4 or
+  later, and is not offered in fullscreen mode, with a layout that leaves no
+  other window (`enew`, `tabnew`), on Windows, or when the key is already
+  used by `g:fzf_vim.paste_key`, `g:fzf_action`, or the command itself, as
+  `CTRL-ALT-X` is in `:Buffers`
 - Bang-versions of the commands (e.g. `Ag!`) will open fzf in fullscreen
 - You can set `g:fzf_vim.command_prefix` to give the same prefix to the commands
     - e.g. `let g:fzf_vim.command_prefix = 'Fzf'` and you have `FzfFiles`, etc.
@@ -173,6 +182,25 @@ command inserts). Customize it with `g:fzf_vim.paste_key`.
 let g:fzf_vim.paste_key = 'alt-enter'
 ```
 
+#### Show key
+
+Most commands support a key that opens the current entry in the window fzf
+was started from and leaves fzf open, so you can look through several entries
+in one run (see the command list above). It is labelled `Show` in the footer
+to set it apart from `Enter Open`, which opens the entry and closes fzf.
+Customize it with `g:fzf_vim.show_key`, or set it to an empty string to turn
+the key off. A comma-separated list binds every key in it. It takes the key
+over from `$FZF_DEFAULT_OPTS`, so pick another key or turn it off to keep a
+binding of your own.
+
+```vim
+" Key to open the current entry, leaving fzf open (default: 'ctrl-o')
+let g:fzf_vim.show_key = 'ctrl-o'
+
+" Use CTRL-O and double-click
+let g:fzf_vim.show_key = 'ctrl-o,double-click'
+```
+
 #### Command-level options
 
 ```vim