Fixes graph view right-click un/group

Right-click group/ungroup was not working properly and repeated use would result in the menu items being permanently greyed out. The problem was that ```lib/cylc/gui/SuiteControlGraph.py:grouping()``` assumed that the ```group``` and ```ungroup``` lists passed to the server graph retrieval function held the current lists of manually grouped and ungrouped nodes. In fact, they are just to specify nodes to *newly* group or ungroup, and the complete list of currently grouped nodes is held server-side (it shouldn't be - see issue #164).