Improvement: Do not force child themes to reimplement any theme settings, but allow it

[danowar2k]

Right now, Boost Union implements a mix of forcing child themes to reimplement some settings and, in case of other settings, explicitly takes Boost Union's settings.

The first case could lead to unnecessary duplication of settings code.
The second case could maybe lead to duplication of function code if one would prefer to implement a theme's settings as "all settings for theme appearance should be configurable in a theme's settings pages).

I propose a function that returns a child theme's setting value if that setting exists in that child theme and otherwise returns Boost Union's setting value.

I'll prepare a pull request with that function (probably in lib.php or locallib.php, even if I like autoloaded classes wrapping this stuff, i.e. "class settingsutils"), and if the idea is accepted, I'd complete the PR with a bigger commit replacing all settings use with using this function.

[danowar2k]

I said somewhere else that it would perhaps be wise to wait to implement this feature until we have some more people that would benefit from this change. So, if anyone approves of this change, I'd ask of you to inform us about your specific use case so that we get a broader understanding of the utility of this.

At our institution, we currently don't need this change because at the moment, we just configure Boost Union and do not need to override any of it's settings. We also decided that it was ok to define our own theme but not have a separate settings page for it, but just configure Boost Union.

Therefore my older PR #250 wasn't needed by us at all.