Add an install target.

[Petross404]

Hi,

I want to install QNanoPainter in my Gentoo/Linux and I wanted an install cmake target.
Therefore, I created this PR.

>>> Merging dev-qt/qnanopainter-9999 to /
--- /usr/
--- /usr/share/
--- /usr/share/doc/
>>> /usr/share/doc/qnanopainter-9999/
>>> /usr/share/doc/qnanopainter-9999/README.md.xz
--- /usr/include/
>>> /usr/include/qnanopainter/
>>> /usr/include/qnanopainter/qnanowindow.h
>>> /usr/include/qnanopainter/qnanowidget.h
>>> /usr/include/qnanopainter/qnanoradialgradient.h
>>> /usr/include/qnanopainter/qnanoquickitempainter.h
>>> /usr/include/qnanopainter/qnanoquickitem.h
>>> /usr/include/qnanopainter/qnanopainter.h
>>> /usr/include/qnanopainter/qnanolineargradient.h
>>> /usr/include/qnanopainter/qnanoimagepattern.h
>>> /usr/include/qnanopainter/qnanoimage.h
>>> /usr/include/qnanopainter/qnanofont.h
>>> /usr/include/qnanopainter/qnanocolor.h
>>> /usr/include/qnanopainter/qnanoboxgradient.h
--- /usr/lib64/
>>> /usr/lib64/libqnanopainter.so
>>> dev-qt/qnanopainter-9999 merged.
>>> Regenerating /etc/ld.so.cache...