KNewFileMenu: Offer custom folder icon directly from new dialog

This adds a strip of folder icons (row of colors and common custom icons) to the new folder dialog to make it more convenient to create folders with a custom icon and to raise awareness that this is actually possible.

An icon that isn't in the default set can be picked through KIconDialog. It is then added to the list, so it will gradually fill with the user's most used custom icons. The "Custom Icon" collapsible group box also remembers whether it was expanded, so users that use this feature get to see it, whereas those who don't won't be bothered by it.

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Default

Custom icon from the list

There’s something odd with the collapsible group box expanded size, the buttons slightly jump/grow when you click them after expanding the group box. I assume something with minimum size vs size hint. Also I couldn’t figure out how to remove the side margin from the grid layout so that the grid aligns with the “Custom Icon…” button below…