Design/Tango refresh 3.6

To continually improve our Tango theme (at least until a new theme is ready for inclusion) ... the following icons were added to replace some icons that were imported from either the Industrial or the Galaxy theme when building.

All of these icons are already included in nightly builds of LibreOffice 3.6.

Formula input
These icons can be seen in:
 * Calc: formula input line
 * Writer: formula input line

Extensions
These icons can be seen in the extension manager.

Writer Toolbar
These icons can be added to toolbars of Writer. The ¶ (non-printing characters) icon is visible by default in the Standard toolbar of Writer.

Start Center
These icons can be in the Start Center. The Open File image is also part of the Standard toolbar of all applications.

Media Player
These icons are visible in the embedded media player and the accompanying media toolbar in all applications. Due to a in LibreOffice, the larger icons are never used, though.