OpenMW-CS: Support for dark/light icons

The current editor icons were made to fit on a bright/light UI theme. If someone is using a dark UI theme the icons are not as visible thus reducing editor's usability. To fix this problem the editor would need to support switching between different icon sets. There are two parts to this:

• support for switching the icon sets in the editor's preferences
• an icon set that fits on dark UIs

I plan to create the icon set based on the already existing icons. This issue is more about having the support in the editor to then use them.