Qt: ByteViewText fixups.

Calculate our X coordinate using floating point arithmetic. This should
hopefully fix the wierd subpixel text shifting for some font sizes.

Add alpha blending convenience routines to ColorUtils. Use them to
create the offset colors. The button color palette wasn't working very
well under Ubuntu.

Use QFontMetricsF::lineSpacing for our line height.

Note that we don't support the BOLD gui.hex_dump_highlight_style
preference. Leave a hint for anyone wishing to do so. Comment out
related code for now.

Change-Id: Ief77c9c8d00e05560cae6ecd781bd309027f6f5a
Reviewed-on: https://code.wireshark.org/review/4927
Reviewed-by: Gerald Combs <gerald@wireshark.org>