Make an easy method to set the minimum QS version requirement for all plugins

As discussed on IRC, I've modified bltrversion so that it looks for a value `QS_BACKWARDS_COMPATIBILITY_BREAK` in Developer.xcconfig and uses this value as the minimum QS version requirement when building any plugins

This is useful for example when we introduce any backwards incompatibilities. E.g. switching from 32/64 bit to 64 bit only can be seen as a backwards incompatibility (plugins build as 64 bit only should not run on 32/64 bit version of QS), so this version is set in the .xcconfig