filter: updates kernel::fft_filter_xxx documentation to better explain how it works and how to use it in other blocks.

Addresses concern raised in issue #662.
cmake: better setup for finding QWT; otherwise, it can be forced to incorrectly "find" non-existent QWT libs.
Merge branch 'maint'
digital: removed some warnings in hdlc_framer.
filter: allow changing sample delay from GRC
filter: fix pfb_arb_resampler tag propagation by setting enable_update_rate