demo: make frame generation (and maximum frame count) a runtime option

demo: make frame generation (and maximum frame count) a runtime option

The previous implementation supported the generation of frames as a
compile time option. This change lets users adjust the feature at
runtime.

In the absence of a frame count limit no frame begin/end markers get
sent (the default behaviour of the previous implementation). When a
frame count limit is specified, the respective number of frames gets
sent and acquisition stops.

The fixed amount of 1000 samples per frame is an arbitrary choice. This
compile time option is easily adjusted in the source code.