[CMakePM] Allow to configure tree view

Commit f76e0de7bc98a3d7171390d682bb5046ddfce634 introduces support for the CMake custom group definitions, that can't be controlled by the existing options:

• "Hide Source and Headers Groups" at the Source Tree filters level
• "Show subfolders inside source group folders" at the CMake settings level

I work on the set of the projects with simple and ugly custom group definitions:

source_group("src" FILES ${MAIN_SRC})
source_group("include" FILES ${MAIN_HEADERS})

for some reasons I can't change it. But it mixes all sources and headers in the single flat list now. At same time most of files are logically grouped in the source tree. As a result using new approach of the new files adding complicated now (adding new files to the target).

I suggest adding CMake option like "Ignore custom group definition" that are turned off by default (to keep current behavior for new or existing users that need it). This option should return behavior that was before f76e0de7bc98a3d7171390d682bb5046ddfce634.

Moreover, I can make all needed changes and submit to Gerrit for review, if team accept this suggestion but has no time to implement it.