sci-mathematics/rstudio: Fix bug 558900, unbundle qtsingleapplication

sci-mathematics/rstudio: Fix bug 558900, unbundle qtsingleapplication

Thanks to Helmut Jarausch for reporting, Franz Fellner for suggesting patching
rstudio to use dev-qt/qtsingleapplication[qt5] rather than the bundled version.
Remove sys-devel/clang upper bound, thanks to Helmut Jarausch for reporting and
testing with clang-3.6.2-r100. I tested it builds with clang-3.7.0_rc3.
Fix bug 558988 sci-mathematics/rstudio-0.99.473 missing qt dependencies,
thanks to Oleg and Anthony Staines for reporting and testing.

Package-Manager: portage-2.2.20.1