qtgui: adds trigger capabilities to time sink.

Mode, slope, level, delay, and channel settable in GRC and through interactive menu.
runtime: fixing tag locations in blocks with history. Have to move the tag by the history value to locate it properly with the item it's associated with.

Also added ability to set the tag propagation delay, which is required for how the blocks::delay works.
runtime: added ability to set group delay in a block.

Right now used to affect where the tags sit after a block with delay.
Updated some blocks for the group_delay concept.

filter: FFT and (decim/interp) FIR filters can set through GRC.

blocks: delay block's group delay is fixed to the delay value.

qtgui: time sinks history is related to the group delay for displaying tags.
qtgui: added ability to trigger off a tag with a specific key.