New functions for dialog position and german translation

We will be incorporating WinSparkle in our product next year. However, we had two requirements which it did not fulfill:
1. We wanted to be able to display the window centered on the screen. I added the function set_dialog_position(x, y) to offer this.
2. Our software is German. We wanted WinSparkle in German. To achieve this I extended it by adding support for wxLocale and the function set_locale(const char* lang). I also provide my translation of the GUI in German as a .po file (in correspondence to the wxWidgets translation guidelines).