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

[Allen Li]

Interestingly, when display-battery-mode and display-time-mode are set
through my custom file, the battery display comes before the time
display.  However, toggling either of them interactively will always
result in the time display coming before the battery display.

The cause of this odd behavior is that display-battery-mode appends
and removes its symbol in global-mode-string when it is toggled on or
off, while display-time-mode only appends its symbol and does not
remove it when it is toggled off.  The reason display-battery-mode
comes first after Emacs starts is because of how the custom file
works; user options are sorted alphabetically and display-battery-mode
comes first, so it is appended first.

Naturally, this has some implications for whether each display uses
leading, trailing, or no space.