[ATL] CStringT fixes
- Added copy constructor and assignment from CSimpleStringT to CStringT
This fixed initialization while using `operator+` in GCC.
```CStringW s = a + b; ```
operator+ operators are defined for CSimpleStringT.
It worked in MSVC because it did implicit conversion of CSimpleStringT to PCXSTR which called appropriate CStringT constructor.
GCC doesn't do such conversions and triggers an error.
- Unified `operator=(CStringT, PCXSTR)` with the rest
svn path=/branches/GSoC_2017/rapps/; revision=75822