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/CMakeLists.txt

@@ -0,0 +1,15 @@
+serenity_component(
+    PartitionEditor
+    TARGETS PartitionEditor
+)
+
+compile_gml(PartitionEditorWindow.gml PartitionEditorWindowGML.h partition_editor_window_gml)
+
+set(SOURCES
+    main.cpp
+    PartitionEditorWindowGML.h
+    PartitionModel.cpp
+)
+
+serenity_app(PartitionEditor ICON app-space-analyzer)
+target_link_libraries(PartitionEditor LibMain LibGUI LibPartition)