GUI improvements: skinning, table rows filter etc.

1. The GUI can now be customized either by directly providing a CSS file or by choosing a predefined theme and adjusting its colors. A light predefined theme and a dark one are currently provided.
2. Searching in tables replaced by a filter-as-you-type functionality.
3. Forward/Backward buttons removed.
4. 'Copy table' and 'Copy cell' now available only from the context menu.
5. Missing logos added in copper-monitoring-client/src/main/resources/org/copperengine/gui/logo.