Move initial values of LyXRC members to the header

Move initial values of LyXRC members to the header

We use a C++11 construct that puts initial values of members along
with their definition. It is a good construct and now the out of line
constructor and the setDefaults() method can go.

This removes the need to define a dummy constructor in tex2lyx and
client.

The only needed change to the rest of code is a change of signature
for the user_(name|email) to return std::string. They are now called
explicitely from the constructor. We now have to include userinfo.h in
LyXRC.h, but this should not be too expensive.