HD44780 driver lacks the check for busy flag

[gen2thomas]

When "fast" writing in 4-bit mode, the display characters becomes corrupted, also an Clear() often not help.
According to page 9, 24, 29 and 33 of the datasheet it is strongly recommended to check the busy flag before next write operation.

[gen2thomas]

After implementing the check of busy flag I found the root cause of the strange behavior. It is caused by concurrent writing using "SendCommand()" and "WriteChar()". After prevention of concurrent writing by using mutex, no busy flag was set anymore. IMO all implemented sleeps are big enough to prevent this. The busy flag check (and the reading of address counter and data register) increases the lines of code strongly.

Therefor I will only provide small improvements in the first step together with #812 .

In case someone needs the read functionality (including the busy flag evaluation) - it is basically implemented on this branch.