SimpleQWT version 6.0

SimpleQWT is targeted at projects that want only core qwt, with fixes and improvements.

• targets reduced computational and memory footprint compared to full-fat version
• fixes for stable axes/canvas while panning/zooming
• fixes and production-ready antialiasing
• some fixes for plot renderer for wysiwyg
• forked from QWT "6.0.0-svn"
• some fixes pulled from 6.1
• Qt5 fixes, legacy cleanup, simplification and streamlining

Initial version as used by NSpec scientific software.

Q/A:

• Why not QWT 6.1?

It is slower and uses more memory according to my tests (mostly Plot/Slider).
Quick review didn't reveal the single culprit for that change.

• Why not merge it with regular QWT or fix full version?

Full QWT is much larger than just core, i don't have enough time for that.
Some changes are very invasive and not possible there at all.
Some simplifications are only possible after removing more elaborate functions.
I hope most minor fixes and legacy cleanups can be ported back to original version,
but that's a lot of work too due to 6.1 changes.