fix issue, then it wasn't possible to remove functional blocks from
project using right click and popup menu.

The problem is that, PopupMenu was called using IDEFrame object and as
a result item in ProjectTree loosed focus and focus went to IDEFrame.
Therefore later functions couldn't detect which item in ProjectTree
was selected.

fix many compilation warnings about static variables used by non-static functions

[CC] plc_main.c -> plc_main.o
/tmp/be/build/plc_main.c:396:40: warning: ?LogBuff? is static but used in inline function ?copy_from_log? which is not static
memcpy((char*)buf + remaining, LogBuff[level], size - remaining);
^
/tmp/be/build/plc_main.c:395:22: warning: ?LogBuff? is static but used in inline function ?copy_from_log? which is not static
memcpy(buf, &LogBuff[level][buffpos], remaining);
^
...

^
/tmp/be/build/plc_debugger.c:825:20: warning: ?retain_offset? is static but used in inline function ?BufferIterator? which is not static
Retain(retain_offset, size, real_value_p);
^
/tmp/be/build/plc_debugger.c:823:47: warning: ?retain_offset? is static but used in inline function ?BufferIterator? which is not static
unsigned int next_retain_offset = retain_offset + size;
^
^

make attribute CFLAGS and LDFLAGS optional and add default empty value

Actually CFLAGS and LDFLAGS are not required and can be empty.
Without default empty value if target platform in project settings was
set to "Linux" CFLAGS and LDFLAGS was initialized with NoneType.
The result was broken build and it wasn't to save/load project with
such settings.

Traceback (most recent call last):
File "/home/developer/WorkData/PLC/beremiz/beremiz/ProjectController.py", line 956, in _Build
if not builder.build() :
File "/home/developer/WorkData/PLC/beremiz/beremiz/targets/toolchain_gcc.py", line 96, in build
Builder_CFLAGS = ' '.join(self.getBuilderCFLAGS())
TypeError: sequence item 0: expected string, NoneType found