extras: copy_plugins.sh: check framework's date before re-copying

Lipo-ing, copying and signing every plugins takes a lot of time, so this commits heuristically checks whether the copy is needed by comparing every input files against the output framework before copying.

On my machine, it reduces the copy duration for the plugins from a total of 34s when every plugin was copied to 2s, not accounting the fact that changing the file on the build target also required XCode copying again towards the device.