Merge pull request #71 from thielj/master

Split-off MetroFramework.Design.dll
Merge pull request #73 from thielj/master

some more small fixes
small changes on style manager and controls
recent fixes and placeholder (prompt) support for textbox
fixed combobox item rendering using a datasource
MetroStyleExtender (new), MetroStyleManager

Added MetroStyleExtender
Rewrote most of MetroStyleManager

Sven: Can you please review use of MetroStyleManager.Clone in
MetroTaskWindow!? It may not be ideal with owner, _parent, etc.