Allow partial manipulation of define symbols

[ruccho]

Currently we have to specify all enabled define symbols at once with PlayerSettingsSetScriptingDefineSymbolsTask. This is irritating to specify similar but bit different sets of define symbols to multiple build schemes.

[ruccho]

Planned:

• Implement a build task for "addition" of the define symbols which can be added to the scheme multiple times
• Using them with Indirect build scheme overrides #4

Not planned:

• Subtraction of define symbols (it will cause ordering problem of configurations)