Added namelessgui::SpriteWidgets to render unit/terrain in qrw::IngameWindow.
Made namelessgui::Widget::visible protected.
Fixed namelessgui::SpriteWidget::draw().

Now only drawing if sprite is visible.
Finished refactoring of qrw::IngameWindow unit and terrain rendering.

qrw::IngameWindow now uses namelessgui::SpriteWidgets for rendering terrain and units.

Closes #18.
Merge pull request #66 from namelessvoid/ingamewindow-improve

Some minor improvments to the IngameWindow.