add distribution license notice to about dialog as GPL requires

add GPLv2 license

add copyright notices to python files where there were missing, that
is required by GPLv2

fix issue that was impossible to remove local instance of functional
block if pou is written in ST or IL

fix highlighting of multiline comments in ST and IL

close #2

fix error if non-latin character was entered in initial value in
data type element

Traceback (most recent call last):
File "/home/beremiz/editors/DataTypeEditor.py", line 575, in OnStructureElementsGridCellChange
value = self.StructureElementsTable.GetValue(row, col)
File "/home/beremiz/editors/DataTypeEditor.py", line 80, in GetValue
return str(value)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-2: ordinal not in range(128)
Traceback (most recent call last):
File "/home/beremiz/editors/DataTypeEditor.py", line 80, in GetValue
return str(value)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-2: ordinal not in range(128)
Traceback (most recent call last):
File "/home/beremiz/editors/DataTypeEditor.py", line 80, in GetValue
return str(value)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-2: ordinal not in range(128)

fix issue with sometimes wrong return code of ProcessLogger


As a result of wrong return code Beremiz gives folowing traceback:
Traceback (most recent call last):
File "./Beremiz.py", line 850, in OnMenu
getattr(self.CTR, method)()
File "/home/developer/WorkData/PLC/beremiz/beremiz/ProjectController.py", line 925, in _Build
IECGenRes = self._Generate_SoftPLC()
File "/home/developer/WorkData/PLC/beremiz/beremiz/ProjectController.py", line 568, in _Generate_SoftPLC
return self._Compile_ST_to_SoftPLC()
File "/home/developer/WorkData/PLC/beremiz/beremiz/ProjectController.py", line 661, in _Compile_ST_to_SoftPLC
C_files.remove("POUS.c")
ValueError: list.remove(x): x not in list

The problem is that both threads (for reading stdout and stderr) call self.Proc.poll(),
that updates internal returncode field. This call is done without any locking and the first thread gets correct result,
but other gets 0 as retval. If 0 gets thread, that afterwards calls callback finish, then wrong return code is returned
to the parent. Now only the thread with a callback polls for the return code, other thread just checked local value.

Additionally function spin() waits now until all threads finish reading their pipes, so the results are always correct.