Apparently, my linux ubuntu setup of QGIS doesn't contain grass gis, although when I initially installed it last week I added grass gis to the command line. Anyway, I typed in once again and installed grass gis 7.0, however while GRASS starts, the plugin QGIS for it doesn't show up. Any ideas?
GRASS 7 is not compatible with QGIS 2.8.
You can follow the development of the new GRASS plugin on http://www.gissula.eu/qgis-grass-plugin-crowdfunding/progress.html.
For 2.8, you will need to install GRASS 6.x.
A late answer:
QGIS 2.14 is compatible with GRASS 7 now. On Windows, the standalone installer provides separate icons for GRASS 7.0.3 integration. For GRASS 6.4.3 support, you have to stick to QGIS 2.12 or older. Windows versions can be installed in parallel without harm.
On Linux, you have the option to choose support for GRASS 6 in the
debian packages, or GRASS 7 in the
ubuntugis packages. The latter is only available up to Ubuntu trusty (running on Linux Mint 17.3 too).