27f9b1c caused a regression: it uses loop_schedule_deferred() to
defer emitting the "enable focus reporting" termcode. But tui_main()
never processes `tui_loop.events`, so the event was never actually
processed.

But fixing (by processing `tui_loop.events`) would bring back the
problem 27f9b1c tried to fix: it still emits the event too soon.

Instead, do a little dance: schedule the event on `main_loop` and then
forward it to `tui_loop`.

NOTE: after this commit, in tmux 2.3 with `focus-events` enabled,
FocusGained is fired on startup and when resuming from suspend.

27f9b1c caused a regression: it uses loop_schedule_deferred() to
defer emitting the "enable focus reporting" termcode. tui_main() never
processes `tui_loop.events` (which loop_schedule_deferred() depends on),
so the event was never actually processed.

But fixing that (by processing `tui_loop.events`) would bring back the
problem 27f9b1c tried to fix: it still emits the event too soon.

Instead, do a little dance: schedule the event on `main_loop` and then
forward it to `tui_loop`.

NOTE: after this commit, in tmux 2.3 with `focus-events` enabled,
FocusGained is fired on startup and when resuming from suspend.

For some reason, enabling focus reporting during terminal setup, causes
slow rendering during Nvim startup on tmux 2.3 with the tmux
`focus-events` option enabled.

To workaround that issue, this commit defers the request.

closes #7649

init.vim:
    call plug#begin('~/.config/nvim/plugged')
    Plug 'morhetz/gruvbox'
    call plug#end()
    set background=light " background light just to see the effect more quickly
    colorscheme gruvbox
.tmux.conf:
    set -g focus-events on
    set-option -ga terminal-overrides ",xterm-256color:Tc"
    set-option -g default-terminal "screen-256color"

Using `script` to record the terminal session (and `vterm-dump` to
post-process the result):

BEFORE this commit:
    ./build/bin/nvim -u NONE{CR}{LF}
    {DECSM 1049}{DECSM 1}{ESC =}
    {CUP *}{ED 2}{DECSM 2004}{DECSM 1004}{CSI 8,44,156 t}{CSI * r}
    {CUP 1,1}
    {CUP *}{ED 2}{DECSM 25}
    {DECRM 25}{CSI 2   q}{CSI 2   q}
    {CUP *}{ED 2}{LF}
    {ESC (B}{SGR *}{SGR 94}~                                                                                                                                                           {CR}{LF}
    ~                                                                                                                                                           {CR}{LF}

AFTER this commit:
    ./build/bin/nvim -u NONE{CR}{LF}
    {DECSM 1049}{DECSM 1}{ESC =}
    {CUP *}{ED 2}{CSI 8,44,156 t}{CSI * r}
    {CUP 1,1}
    {CUP *}{ED 2}{DECSM 2004}{DECSM 1004}{DECSM 25}
    {DECRM 25}{CSI 2   q}{CSI 2   q}
    {CUP *}{ED 2}{LF}
    {ESC (B}{SGR *}{SGR 94}~                                                                                                                                                           {CR}{LF}
    ~                                                                                                                                                           {CR}{LF}
    ...