Make MusicBrainz support compatible with ApplicationWindow support

Created new branch wip/musicbrainz-appwin-support
Rebased it over wip/application-window
Modified Makefile.am and configure.ac to make it mergable
Use EtApplicationWindow and its BrowserList

Modified musicbrainz_dialog.c to use MainWindow's BrowserList and its functions
Removed all functions related to standalone global variable BrowserList
Corrected mbentityview.c
Added MusicBrainz Menu and Toolbar Item

MusicBrainz Search has been added to Toolbar and Menubar
show-musicbrainz is the action name
on_show_musicbrainz is the action callback
Set Images from resource

Use gtk_image_new_from_resource to set image as an icon to toolbar buttons