bug#30056: 25.3; battery-mode-line-string missing leading space

[Eli Zaretskii]

> From: Glenn Morris <rgm@gnu.org>
> Cc: Allen Li <vianchielfaura@gmail.com>,  30056@debbugs.gnu.org
> Date: Wed, 10 Jan 2018 12:06:47 -0500
> 
> M-x display-time-mode
> M-x display-battery-mode
> 
> -> 9.05AM 0.45 Mail[100.0%]
> 
> See also https://debbugs.gnu.org/18164

We don't do this consistently in the modes which use
global-mode-string: some of them leave a blank at the beginning,
others (the majority, AFACT) don't.  There's not much space on the
mode line, so I'm not sure which way is better.

But if we want to have a separation there, would it make sense to do
this in bindings.el, so that global-mode-string is always separated by
a blank from the preceding text, and modes don't have to remember this
gork?