Added MusicBrainz dialog

Added libmusicbrainz compilation flags. Added musicbrainz_dialog.ui
containing MusicBrainz dialog. Added new file mbentityview.c containing
EtMbEntityView widget. MusicBrainz Search has been added to the toolbar
and menubar show-musicbrainz is the action name.
Implemented EtMbEntityView

Modified musicbrainz_dialog.ui to create place for EtMbEntityView in the
dialog. Implemented different functions of EtMbEntityView in
mbentityview.c. Added EtMbEntityView in dialog in musicbrainz_dialog.c
Implemented asynchronous manual search

Created a base library for MusicBrainz search in mb_search.c and
mb_search.h. Implemented manual search functions in
musicbrainz_dialog.c. Added support for displaying information about
Mb5Entity in mbentityview.c.
FIXME (split) Implemented Select All, Unselect All buttons. Added Artists and Type for Releases.

Added buttons in musicbrainz_dialog.ui. Added callbacks for buttons in
musicbrainz_dialog.c. Added functions for above features in
mbentityview.c. Added Red Line support in mbentityview.c and
mb_search.c.
Implemented Invert Selection and Toggle Red Lines

Added buttons for Invert Selection and Toggle Red Lines in
musicbrainz_dialog.ui. Added callback functions for above buttons in
musicbrainz_dialog.c. Used GtkFilterTreeModel for Toggle Red Lines in
mbentityview.c. Added functions for Toggle Red Lines & Invert Selection
in mbentityview.c.
Implemented selection Up and Down

Added buttons for Up and Down in musicbrainz_dialog.ui. Added functions
for above features in mbentityview.c & mbentityview.h. Added callbacks
for functions in musicbrainz_dialog.c.
FIXME (split) Added more verbose Status Bar Messages and comments

Added function et_show_status_msg_in_idle to show messages in Status Bar
in a thread safe manner. Added more messages depicting more information.
Make TreeView multi selectable

Modified GtkTreeView in EtMbEntityView to select more than one rows.
Implemented Cancellation of Manual Search using GCancellable

Added various cancellation checks in all the modified files
Created a new GCancellable and pass it to every GSimpleResult thread function
Added a Stop button to allow cancellation
Double clicking on Red Line will give already downloaded results

Show the already downloaded stored results instead of redownloading them
everytime.
Coding Style Improvements
Removed FIXME. Added some more comments
Implemented Search Selected Files

Added selected_find_thread_func which will search over all selected albums
Added selected_find_callback responsible for displaying the results.
Similar to Manual Search
Implement DiscID Search

Added support for searching DiscID in mb_search.c. Added dependency for
libdiscid in configure.ac. Added support for displaying DiscID Search
results in mbentityview.c. Added DiscID search's callback and
thread_func in musicbrainz_dialog.c.
Added Test for MusicBrainz Search

Added "make test" in Makefile.am
Added tests cases for MusicBrainz search in test-musicbrainz.c
Added a test soup-server in test-musicbrainz_server.c
Added different xml files containing MusicBrainz retrieved data for server
FIXME (split) Coding Style Improvements and Converted Strings to Translatable Strings
Merged MusicBrainz test server and client into one

Client and MusicBrainz test server are merged into one file
test-musicbrainz.c. Added DiscID test.
Use MB_ENTITY_KIND

Change MB_ENTITY_TYPE_* to MB_ENTITY_KIND_*
FIXME (split) Made musicbrainz optional and added Close button

Made musicbrainz support optional in configure.ac. Added Close button.
Pressing Return/Enter in Manual Search Combo Box Text will start the manual search

Added callback for Entry of Search ComboBoxText on pressing enter.