Fix send box not expanding correctly, and fix layouting in custom button dialog

Sends M21 whenever SD Upload or SD Print are used.

Automatically attempts to initialise the SD card whenever either of the SD
card related buttons are used.

Allow the graph to expand

This cleans up the layouting code for the GUI

Added Current Z-Height to the status message.

Also added "Print Began at: HH:MM:SS"
and added the message "Print Ended at: HH:MM:SS: and took...

Improved status line estimated time remaining.

Fixed the time estimator in pronsole.py

at beginning of move set feedrate to average of last and current feedrate.  do move with that feedrate.
then set the feedrate to the current feedrate.
if it's just a feedrate change, then the feedrate will become the feedrate.
if there is no feedrate in current move, feedrate will be last feedrate
if there is a move and a new feedrate then the move will take however long the average feedrate would take.

estimation is a bit more optimistic than pessimistic.  however it seems to be a lot more accurate.

uses format:
SD Printing: pp.p% | Est: hh:mm:ss of hh:mm:ss of hh:mm:ss remaining
Printing: pp.p% | @ Line# xxx of yyyyy lines | Est: hh:mm:ss of hh:mm:ss remaining

Conflicts:
	stlview.py

Added an Estimated Time Remaining of Total Time to the status bar.

for non-SD Printing. They get more and more accurate until the end.

React to resend only when the command is in the beginning of a line.

Fix bug where second and subsequent file load would cause an error and refuse to load further files. Closes #102