Context menu, nested sub-menus and some styling

Context menus are somehow effective now.

depTree, morph and relation expose information to it. The depTree allows you to move a token to root, or disconnect it (remove it's edge), morph displays morph info and relation allows to change the label.

The label change is achieved by the `labelSelector` directive that makes use of a pure css submenu structure and two other nested directives. At least one step closer to nested tagsets etc. We'll see which direction this eventually takes.

Minor styling has been done for several parts of the application, especially regarding it's compatibility with low resolution screens. Not perfect so far, but a start.