Spreadsheet: Add an import wizard, and add support for custom CSV files

Fixes the import half of #4269.

Userland/Applications/Spreadsheet/select_format_page.gml

@@ -0,0 +1,33 @@
+@GUI::Widget {
+    name: "select_format"
+
+    layout: @GUI::VerticalBoxLayout {
+        margins: [20, 20, 20, 20]
+    }
+
+    @GUI::Label {
+        text: "Please double-check the guessed file type\nor select the correct one below"
+        text_aliignment: "TopLeft"
+        fixed_height: 32
+    }
+
+    @GUI::Widget {
+        fixed_height: 25
+
+        layout: @GUI::HorizontalBoxLayout {
+        }
+
+        @GUI::Label {
+            text: "Type: "
+            autosize: true
+        }
+
+        @GUI::ComboBox {
+            name: "select_format_page_format_combo_box"
+            model_only: true
+        }
+    }
+
+    @GUI::Widget {
+    }
+}