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/PartitionModel.cpp

@@ -0,0 +1,88 @@
+/*
+ * Copyright (c) 2022, Samuel Bowman <sam@sambowman.tech>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <Applications/PartitionEditor/PartitionModel.h>
+#include <LibPartition/EBRPartitionTable.h>
+#include <LibPartition/GUIDPartitionTable.h>
+#include <LibPartition/MBRPartitionTable.h>
+
+namespace PartitionEditor {
+
+String PartitionModel::column_name(int column) const
+{
+    switch (column) {
+    case Column::Partition:
+        return "Partition";
+    case Column::StartBlock:
+        return "Start Block";
+    case Column::EndBlock:
+        return "End Block";
+    default:
+        VERIFY_NOT_REACHED();
+    }
+}
+
+GUI::Variant PartitionModel::data(GUI::ModelIndex const& index, GUI::ModelRole role) const
+{
+    if (role != GUI::ModelRole::Display)
+        return {};
+    if (!m_partition_table)
+        return {};
+
+    auto optional_partition = m_partition_table->partition(index.row());
+    if (optional_partition.has_value()) {
+        auto partition = optional_partition.release_value();
+
+        switch (index.column()) {
+        case Column::Partition:
+            return index.row() + 1;
+        case Column::StartBlock:
+            return partition.start_block();
+        case Column::EndBlock:
+            // FIXME: Either MBR end block is off by one (if supposed to be exclusive bound)
+            //        or GPT end block is off by one (if supposed to be inclusive bound).
+            //        This is an issue in LibPartition.
+            return partition.end_block();
+        default:
+            VERIFY_NOT_REACHED();
+        }
+    }
+
+    return {};
+}
+
+ErrorOr<void> PartitionModel::set_device_path(String const& path)
+{
+    auto file = TRY(Core::File::open(path, Core::OpenMode::ReadOnly));
+
+    auto mbr_table_or_error = Partition::MBRPartitionTable::try_to_initialize(file);
+    if (!mbr_table_or_error.is_error()) {
+        dbgln("Found MBR partition table on {}", path);
+        m_partition_table = move(mbr_table_or_error.value());
+        invalidate();
+        return {};
+    }
+
+    auto ebr_table_or_error = Partition::EBRPartitionTable::try_to_initialize(file);
+    if (!ebr_table_or_error.is_error()) {
+        dbgln("Found EBR partition table on {}", path);
+        m_partition_table = move(ebr_table_or_error.value());
+        invalidate();
+        return {};
+    }
+
+    auto guid_table_or_error = Partition::GUIDPartitionTable::try_to_initialize(file);
+    if (!guid_table_or_error.is_error()) {
+        dbgln("Found GUID partition table on {}", path);
+        m_partition_table = move(guid_table_or_error.value());
+        invalidate();
+        return {};
+    }
+
+    return Error::from_errno(ENOTSUP);
+}
+
+}