Changed behaviour on error

Free downloaded nodes in case of an error during search operation.
Achieved it by creating a new parent node in ThreadData of Search,
then setting this node as new parent node for EtMbEntityView.
Fixed Warning Messages
Implemented Fetch More Data operation

Added Fetch More Data Button in musicbrainz_dialog.ui
Added support for offset and fetching more data in mb_search.c
Changed Search in musicbrainz_dialog.c to get offset & use it to fetch more data
Changed mbentityview.c to fetch more data for AutomaticSearch
Bug Fixed: pressing close when searching will give a crash

Perform checks in search callbacks before displaying downloaded data
Added Country and Date columns for Album
Make libdiscid support optional
Changed Retrieving Track message to include medium also
Use NULL pointer check instead of GTK_IS_TYPE
Toolbar buttons are disabled at the starting
Use et_music_brainz_dialog_clear

et_music_brainz_dialog_clear will unref and set dialog to NULL
Now IS_ET_MUSICBRAINZ_DIALOG check is not required
Use g_return_if_fail() for checking GtkTreeModelFilter
Use default Release if given in Applying Tags

If Tracks are obtained from a parent Album
then use that Album's tag for Applying Tags
Give choices to user to select the Tag only
if there are more than one choice of Album Tags & Tracks are standalone
Show File Path in TagChoiceDialog
Fixed Memory leaks with mbentityview.c

free the string obtained from gtk_list_store_get_values
FIXME (split) Fixed memory leak in free_mb_tree_node

Added g_slice_free statements, don't know how it got deleted
ApplyTAGs also add Disc and Track Tags

Get Disc and Track tags from recording.
A recording contains Medium and Track information.
Use that information in et_mb5_recording_get_medium_track_for_release
Updated TagChoiceDialog to display Disc and Track tags
If there are more than one Disc and Track tags for recording then
display choices for them
Apply TAGs add Composer Tag

Use "artist-rels" to get Composer Tag.
If available Composer Tag will also be applied.
Use et_mb5_recording_get_composers to get all composers
Added comments to mb_search.c
Added comments in mbentityview.c
Added comments to musicbrainz_dialog.c