Allow to set app id from command line

[sloonz]

Currently cage has "wlroots" as its app id. Add a flag to make it configurable.

My use-case : I want some cage instances to be floating, others not to be floating. This patch allows me to do that using for_window in sway config :

for_window {
        [app_id="cage:floating:.*"] floating enable
}

And running cage -i cage:floating:xxx cmd.

[joggee-fr]

Related to issue #400.
@sloonz, did you try also try to "transfer" app_id from main application just like it is done for title?

[sloonz]

I think we want to be able do differentiate between for example a caged chromium and an non-caged chromium ?

[joggee-fr]

Ping @emersion.
Or is it something that should be set in a WLR_* environment variable for wlroots?