RackType::addPlugin returns a boolean indicating if the plugin was added

[drowaudio]

Added here: https://github.com/Tracktion/tracktion_engine/commit/fa5eb48c43f61b6539f9057e95cc0d8bbca170bf

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Unit Test Results

       8 files  ±0         8 suites  ±0   21m 49s :stopwatch: + 1m 5s    111 tests ±0     111 :heavy_check_mark: ±0  0 :zzz: ±0  0 :x: ±0  6 244 runs  ±0  6 244 :heavy_check_mark: ±0  0 :zzz: ±0  0 :x: ±0 

Results for commit fa5eb48c. ± Comparison against base commit 6c65b3dd.

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Performance Test Results

0 files  ±0  0 suites  ±0   0s :stopwatch: ±0s 0 tests ±0  0 :heavy_check_mark: ±0  0 :zzz: ±0  0 :x: ±0 

Results for commit fa5eb48c. ± Comparison against base commit 6c65b3dd.