[kcms/keyboard] Redesign "Add Layout" dialog

Instead of having a three-combobox design of language+layout+variant, it now has a single list of layout variants and a search box.

Before	After

Why:

The new design is far simpler, and IMO much better reflects how people actually look for layouts: they don't usually know in which "Layout" category their language is going to be, and a lot of major languages don't even appear in the "Language" combobox (like Hindi or Telugu, for example). A lot of users probably thought their language just wasn't supported because they looked for it in the "Language" combobox and didn't find it, because they didn't know it wasn't an exhaustive list of all languages offered in all the variants. It was just misleading.

With the new design, they just type in the name of their language and done.

It's also far less code.

TODO:

• QListWidget->iconSize() returns QSize(-1, -1), so we can't create a proper empty icon, so layouts without flag icons look misaligned. As far as I know it used to work with QComboBox->iconSize().

cc @teams/usability