v3.0.0-rc.7 UI Refresh

New this release:

• Add album track duration (#433)
• Merged fixes from hotfix-3-0-0-rc-6:
  • enhanced Discogs album match in Library Editor (#430)
  • fixed handing of '??' in playlist entry (#429)
  • added 'General - Experimental' subcategory (cec3e3d)
  • suppress 'on-now' album name if none (3a3666a)
  • finessed narrow navigation layout (35fcc8f)

Important: If you are upgrading from a release prior to v3.0.0-rc.1, run the script db/convert_v2_11_7_to_v3_0_0.sql. If you are upgrading from an earlier release candidate, run the following database commands:

 ALTER TABLE `tracknames` ADD COLUMN `duration` time DEFAULT NULL; 
 ALTER TABLE `colltracknames` ADD COLUMN `duration` time DEFAULT NULL;