cairomm: new package

[Thomas:
 - rename package from libcairomm to cairomm in order to match
   upstream name.
 - put the package next to cairo in menuconfig
 - add missing dependency on libsigc
 - add missing depends on BR2_INSTALL_LIBSTDCPP
 - fix license: it's LGPLv2+, not LGPLv2.]

Signed-off-by: James Knight <james.knight@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>