Header warning fixes and build cleanup

[sgaist]

This pull request fixes the public headers include warnings generated when compiling the module as well as a private header missing "warning" warning.

It also cleans up the module build setup.

As a side effect, that makes the module build on Windows properly with VS2015.

[VSRonin]

I'm not completely sure about this. The whole thing is deployed to work as a Qt Module, make install does that.
QT += xlsx
Does the include and linking work for you with no include warnings.

I personally don't like to have external libraries work this way but it's the original author choice

[sgaist]

If you mean, can I build an application without warning ? Yes.

What I fixed with this PR is a series of warning generated when building a Qt module with "unclean" headers.

In the absolute, the standard Qt public headers are also written differently when using classes from other submodules (i.e. <QtModule/qclassname.h>) but I wanted to let that for another request.

This simplifies the life of people not using qmake to handle their project.

[hongnod]

I made a shared lib fork--https://github.com/topillar/QtXlsxWriter, I don't like module way either, but it hard to follow other's commits.