LibGfx: Unify Rect, Point, and Size

This commit unifies methods and method/param names between the above
classes, as well as adds [[nodiscard]] and ALWAYS_INLINE where
appropriate. It also renamed the various move_by methods to
translate_by, as that more closely matches the transformation
terminology.

Userland/Libraries/LibGfx/DisjointRectSet.cpp

@@ -56,7 +56,7 @@ void DisjointRectSet::shatter()
 void DisjointRectSet::move_by(int dx, int dy)
 {
     for (auto& r : m_rects)
-        r.move_by(dx, dy);
+        r.translate_by(dx, dy);
 }
 
 bool DisjointRectSet::contains(const IntRect& rect) const