add helpers to create service panes

a service pane is considered to run a persistent process like log
output, a web server or guard.
it defaults to starting minimized to 2 cells and not capturing process
output.
the command MaqueAddService provides convenient service creation.
improve the pane creation prevention guard
move tmux commands to plugin/tmux.vim
don't repack the layout when closing panes while exiting
consider minimized stretchable panes as fixed-size when packing layout
remove debug statement
Schedule layout actions for later execution if layout hasn't been done yet

Calls to add_service() and add_pane_in_layout() will now be postponed
until the function maque#initialized() evaluates to 1.
Execution of the scheduled calls is done in
maque#util#run_scheduled_tasks(), which is executed in
after/plugin/tmux.vim after all panes have been created.
move check for g:maque_tmux_panes_created to after/plugin/tmux.vim
use add_pane_in_layout for default pane creation
MaqueAddCommand convenience command
don't autoclose services
move tmux mappings to plugin/tmux.vim
fix layout minimization

provide effective_size() for Layout, don't apply
non-layout-direction-size
compensate for layoutless setup when minimizing