Use utf8 for text pasted from the clipboard

Change the g_Windowing paste function to return a utf8 string. I've also renamed the function to GetClipboardText to make it clear what is being returned (we might want to paste other objects, e.g. media files, in the future.

I've used g_charsetConverter to do the utf8 to unicode conversions. This means using a CStdStringA in place of std::string.

I'm not as comfortable as I could be with the unicode to utf8 conversions and back, but I've tested by pasting unicode (Cyrillic) characters, both on Win32 and OSX, and it works fine.