Probe encryption mappings as needed using blkid (#148)

GParted no longer recognises file systems inside LUKS encryption, apart
from the few recognised by GParted's internal detection.  Bisected to
this commit:
    8b35892ea5
    Pass device and partition names to blkid (#131)

Prior to this commit blkid was run querying all known block devices
including active encryption mappings, hence prior recognition.  With
this commit blkid was run only for named or found disk devices and
associated found partitions from /proc/partitions, so no more
recognition of encrypted file systems.

Fix by running blkid on the encryption mapping just before querying for
the file system.  This restores the level of functionality that existed
before.

Closes 148 - Encrypted file systems are no longer recognised

src/GParted_Core.cc

@@ -1072,6 +1072,11 @@ void GParted_Core::set_partition_label_and_uuid( Partition & partition )
 FSType GParted_Core::detect_filesystem_in_encryption_mapping(const Glib::ustring& path,
                                                              std::vector<Glib::ustring>& messages)
 {
+	// Run blkid identification on this one encryption mapping.
+	std::vector<Glib::ustring> one_path;
+	one_path.push_back(path);
+	FS_Info::load_cache_for_paths(one_path);
+
 	FSType fstype = FS_UNKNOWN;
 
 	PedDevice *lp_device = NULL;