Add new layout manager XfdashboardDynamicTableLayout to fix issue #28

XfdashboardDynamicTableLayout is a layout manager which layouts children in a dynamic table grid. Rows and columns are inserted and deleted automatically depending on the number of visible child actors. Usually it will prefer more columns over rows.
Clean up code of XfdashboardDynamicTableLayout

Make nice-code(tm) of XfdashboardDynamicTableLayout ;)
Optimize allocation function in XfdashboardDynamicTableLayout
Allow some ClutterActor properties to be stylable

Allow the properties '{x,y}-expand' and '{x,y}-align' to be themable. This addresses issue #28 to avoid that a cell in XfdashboardDynamicTableLayout is always filled completely by child (and its background). To avoid this set 'x-expand' and/or 'y-expand' to 'false' - maybe also set '{x,y}-align' accordingly to position child in available allocation space (e.g. set to 'center').
Fixed wrong cast to ClutterFlowLayout but XfdashboardDynamicTableLayout is used
Also use new XfdashboardDynamicTableLayout in search view
Remove unused variables

Remove unused variables from XfdashboardDynamicTableLayout to avoid compiler warnings
Post-release version bump to 0.2.1
Fix coding style
Improve search speed a little bit

Improve search speed by caching loaded .desktop files. Before this commit each time the result list is retrieved, each item' .desktop file was loaded, parsed and additional information retrieved. Then this result list was sorted what caused the .desktop files being loaded, parsed etc. or each item. To avoid this each .desktop loaded will be cached so it will be only loaded once. At my slowest machine this reduced the mean search time for the term 'a' from 350ms to 240ms.