LibGfx+Everywhere: Change Gfx::Rect to be endpoint exclusive

Previously, calling `.right()` on a `Gfx::Rect` would return the last
column's coordinate still inside the rectangle, or `left + width - 1`.
This is called 'endpoint inclusive' and does not make a lot of sense for
`Gfx::Rect<float>` where a rectangle of width 5 at position (0, 0) would
return 4 as its right side. This same problem exists for `.bottom()`.

This changes `Gfx::Rect` to be endpoint exclusive, which gives us the
nice property that `width = right - left` and `height = bottom - top`.
It enables us to treat `Gfx::Rect<int>` and `Gfx::Rect<float>` exactly
the same.

All users of `Gfx::Rect` have been updated accordingly.

Userland/Libraries/LibGUI/Tray.cpp

@@ -82,7 +82,7 @@ void Tray::paint_event(GUI::PaintEvent& event)
         icon_rect.center_vertically_within(rect);
 
         Gfx::IntRect text_rect {
-            icon_rect.right() + 5,
+            icon_rect.right() + 4,
             rect.y(),
             rect.width(),
             rect.height(),