PartitionEditor: Add the beginnings of a partition editor :^)

This adds a new application PartitionEditor which will eventually be
used to create and edit partition tables. Since LibPartition does not
know how to write partition tables yet, it is currently read-only.

Devices are discovered by scanning /dev for block device files.
Since block devices are chmod 600, PartitionEditor be must run as root.

By default Serenity uses the entire disk for the ext2 filesystem
without a partition table. This isn't useful for testing as the
partition list for the default disk will be empty. To test properly,
I created a few disk images using various partitioning schemes
(MBR, EBR, and GPT) and attached them using the following command:

export SERENITY_EXTRA_QEMU_ARGS="
  -drive file=/path/to/mbr.img,format=raw,index=1,media=disk
  -drive file=/path/to/ebr.img,format=raw,index=2,media=disk
  -drive file=/path/to/gpt.img,format=raw,index=3,media=disk"

Userland/Applications/PartitionEditor/PartitionEditorWindow.gml

@@ -0,0 +1,26 @@
+@GUI::Widget {
+    fill_with_background_color: true
+    layout: @GUI::VerticalBoxLayout {}
+
+    @GUI::ToolbarContainer {
+        @GUI::Toolbar {
+            layout: @GUI::HorizontalBoxLayout {
+                margins: [0, 4]
+            }
+
+            @GUI::Label {
+                text: "Device: "
+                autosize: true
+            }
+
+            @GUI::ComboBox {
+                name: "device_combobox"
+                fixed_width: 100
+            }
+        }
+    }
+
+    @GUI::TableView {
+        name: "partition_table_view"
+    }
+}