Keyboard shortcuts for "full-page" scroll, and bitmap screenshot capture

[sdbbs]

The changes/commits in this branch introduce three new keyboard shortcuts:

• CTRL + m / CTRL + n

Scroll forward or backward (right or left) in time (horizontally) for the
entire width of the capture, currently shown in the PulseView window viewport,
according to current zoom settings (could be called a "full-page" scroll, by
analogy with printable document viewers)

• CTRL + b

Save a bitmap screen capture (screenshot) of only the ruler and the viewport,
currently shown in the PulseView window; which allows these screenshots to be
stitched together horizontally afterwards, if they are obtained through a
"full-page" scroll (note: not pixel-perfect). Screenshots are saved with a
filename of 'pulseview_yyyyMMdd_hhmmss.png' (UTC timestamp), in the
temporary directory of the OS.

With these shortcuts, one can export a series of screenshots at arbitrary zoom level, and then stitch them together, using e.g.:

magick montage pulseview_* -tile x1 -geometry +0+0 pulseout.png

For an example, see https://imgur.com/a/quqLdPE - here the old examples/ad5258_read_once_write_continuously_triangle.sr was loaded, the analog channel was interpreted as digital, and UART decoder was ran on it (a useless example in reality, however it tests most of the graphics we might expect to find in PulseView). (EDIT: for this kind of operation, one probably wants to enable PulseView Settings/Decoders/Always show all rows, even if no annotation is visible)