LibGUI: Rewrite layout system in terms of min and max sizes

This patch removes size policies and preferred sizes, and replaces them
with min-size and max-size for each widget.

Box layout now works in 3 passes:

    1) Set all items (widgets/spacers) to their min-size
    2) Distribute remaining space evenly, respecting max-size
    3) Place widgets one after the other, adding spacing in between

I've also added convenience helpers for setting a fixed size (which is
the same as setting min-size and max-size to the same value.)

This significantly reduces the verbosity of widget layout and makes GML
a bit more pleasant to write, too. :^)

Applications/PixelPaint/FilterParams.h

@@ -82,7 +82,6 @@ private:
             for (size_t column = 0; column < columns; ++column) {
                 if (index < columns * rows) {
                     auto& textbox = horizontal_container.template add<GUI::TextBox>();
-                    textbox.set_preferred_size({ 30, 50 });
                     textbox.on_change = [&, row = row, column = column] {
                         auto& element = m_matrix.elements()[row][column];
                         char* endptr = nullptr;