Variables: made set modifiers descendants of the sieve object too.

src/lib-sieve/plugins/variables/Makefile.am

src/lib-sieve/plugins/variables/cmd-set.c

src/lib-sieve/plugins/variables/ext-variables-common.c

src/lib-sieve/plugins/variables/ext-variables-common.h

src/lib-sieve/plugins/variables/ext-variables-modifiers.c

src/lib-sieve/plugins/variables/ext-variables-modifiers.h

src/lib-sieve/plugins/variables/ext-variables.c

src/lib-sieve/plugins/variables/sieve-ext-variables.h

src/lib-sieve/plugins/variables/variables-match.sieve

src/testsuite/tests/extensions/variables/basic.svtest