qtgui: updating how freq message ports work.

Use absolte frequencies relative to sample rate and center freq.

Double-click does not retune, just sends message. Feedback to input freq can be used to retune.
docs: Corrects or expands documentation comments for various blocks where I have had to read the source code to understand the

- char_to_short and short_to_char scale by a factor of 1<<8.
- What delay actually does.
- File descriptor blocks close their FDs when destroyed.

And one code change:

The firdes transition width check error text is made consistent with other similar messages.

Addresses Issue #686.
Merge branch 'maint'