DO not clear sent lines dict after each ok - fix for firmware sending resend after ok triggering keyerror

initial hack to accept Slic3r SVG

Fix print button, add tooltips, add icons to layer preview

Ignore "wait" messages from spammy firmwares and show timestamps with received messages (courtesy of juri_)

Use system default for the background color of plater/compose function.

Update stlview.py   Double-click on window attempted to load a potentially non-existant stl file. Now prints the location and window name.

Closes #174 - Correct layer-count when z is lifted for travel moves

Merge branch 'experimental' of https://github.com/jezmy/Printrun into experimental

Conflicts:
	gviz.py

Layer info display merge

GCode viewer now displays layer info in status bar when scrolling