Use theme for common actions

Added tango icon svgs as fallback
Added tango and awesome icons

Additionally the user can now use the preferences menu to switch between
them at runtime.
Add message box icons
Use tango theme by default

In order not to have no icons on windows upon selecting the default icon theme I
had to define a default theme.
Merge branch 'bug-18160-use-icon-themes'

Conflicts:
	Plugins/org.mitk.gui.qt.ext/src/QmitkExtWorkbenchWindowAdvisor.cpp