build: make update button optional

[worldofpeace]

In environments that don't have Appcenter
or will never have a use for it this button
would be confusing.

[danirabbit]

Instead of a compile-time option how about checking to see if AppCenter is installed at construction time?

[worldofpeace]

Huh, I thought a compile time option was fine.
Checking if it's installed might be less intrusive since you don't have to do any build configuration. I'm not sure I'd make that change though.

[danirabbit]

superseded by #98

[worldofpeace]

superseded by #98

💖 Another downstream patch gone.