xdot: add support for text based node search
Enhance text search and move searchbox to toolbox

I made some modifications to awesome text-search.
It now matches sub-strings.
When the searched text matches more than one node, all matching nodes are highlighted.
When there is just one match, pressing the return-key centers the node.
And I moved the search box into the toolbar.
Added a "F" keyboard shortcut to reach easily the search box.