cmTarget: Handle NO_SYSTEM_FROM_IMPORTED.

This handling was lost in commit faedd2be (cmTarget: Fix system include
annotation propagation., 2014-01-01).
Help: Fix link to MAP_IMPORTED_CONFIG_<CONFIG>
cmGeneratorTarget: Add GetObjectSources method.

This method and others like it will be able to get a config parameter
later to implement the INTERFACE_SOURCES feature.
cmGeneratorTarget: Add GetObjectName method.
cmGeneratorTarget: Add HasExplicitObjectName method.
cmGeneratorTarget: Add GetResxSources method.
cmGeneratorTarget: Add GetIDLSources method.
cmGeneratorTarget: Add GetExternalObjects method.
cmGeneratorTarget: Add GetHeaderSources method.
cmGeneratorTarget: Add GetExtraSources method.
cmGeneratorTarget: Make the SourceEntries private.
cmGeneratorTarget: Add GetCustomCommands method.
cmGeneratorTarget: Add GetExpectedResxHeaders method.
cmTarget: Make GetSourceFiles populate an out-vector parameter.

In a future patch, this will also be populated with extra
sources from the linked dependencies.
Merge topic 'minor-cleanups' into next

c9836c8 cmTarget: Make GetSourceFiles populate an out-vector parameter.
aa6f674 cmGeneratorTarget: Add GetExpectedResxHeaders method.
137e970 cmGeneratorTarget: Add GetCustomCommands method.
e975704 cmGeneratorTarget: Make the SourceEntries private.
9c859c5 cmGeneratorTarget: Add GetExtraSources method.
d251f2d cmGeneratorTarget: Add GetHeaderSources method.
9e5909d cmGeneratorTarget: Add GetExternalObjects method.
65deeb8 cmGeneratorTarget: Add GetIDLSources method.
bb0e476 cmGeneratorTarget: Add GetResxSources method.
81b9a32 cmGeneratorTarget: Add HasExplicitObjectName method.
c26aad4 cmGeneratorTarget: Add GetObjectName method.
7654870 cmGeneratorTarget: Add GetObjectSources method.
f579fe0 Help: Fix link to MAP_IMPORTED_CONFIG_<CONFIG>
590d238 cmTarget: Handle NO_SYSTEM_FROM_IMPORTED.