fix modplug support

Fix build error due to passing a const pointer to modplug read function.
Fix UI baseclass (QWidget instead of QDialog)
Fix UI size behavior for ModPlug Decoder Preferences.
Remove superfluous tabstops that were giving warnings.

Signed-off-by: Stefan Nuernberger <kabelfrickler@gmail.com>
Merge pull request #159 from snue/feature/modplug

fix modplug support