Activate LiquidTWI2 device detection and space-pad LCD status line.

This pull request (my first, please be gentle) comprises of two parts:

1) Activation of LiquidTWI2's LCD detection feature by passing of an additional flag to LiquidTWI2's LCD class constructor.
   The introduction of an lcd_detected function to return the LiquidTWI2::LcdDetected class function result for supported chipsets or always true for other LCD implementations or the case where LCD support is disabled.
   An early return from the M0 handler in the event that LCD support is compiled in but no LCD panel is attached. This prevents a hang where the code is waiting for input from the LCD panel pushbutton.

2) Status messages sent to the status line of the LCD are overlaid on previous content. If the previous content is longer then it persists and confuses the newly presented message. This change pads the remaining characters after the new message with spaces and null terminates the string.

Tony Lock (of T3P3) has tested changes relating to (1) on RAMPS and Sanguinololu and I have tested both (1) and (2) on Melzi.