While we have to keep these conversions enabled by default, they are very
dangerous as they can result in silent data loss on any system not using a
locale with UTF-8 encoding, i.e. always under MSW.
Allow mitigating this by defining wxNO_UNSAFE_WXSTRING_CONV when compiling the
application code using the library, which makes these conversions invisible to
the user code, and so can be used without recompiling the library.
Also add wxUSE_UNSAFE_WXSTRING_CONV which can be set to 0 when compiling the
library to disable these conversions globally for all applications using it.
Closes#11830.
e125c3b657