Refactor code for displaying track lists.

[jcass77]

This PR represents the start of an effort to consolidate how lists of tracks are retrieved and displayed in the UI:

• fixes an issue that caused the popup menu in the browse pane to render differently than the others, or not at all. Fixes Can't use contextmenu in browse pane #126.
• further refactoring of renderSongLi. There's more work to be done here as the various play... functions can also probably be refactored and consolidated. I'd eventually like to get to a point where we only have one list of tracks that can be cached and used in all of the panes. But that is a PR for another day.
• fixed a small issue with the default images for artists and albums sometimes not displaying if they cannot be retrieved from last.fm.
• browsing using the local or file extensions will now also render album and artist details, which fixes Track artist is not displayed in browse results #99. It works well but the performance impact is noticeable for lists containing more than a hundred tracks or so. We might eventually either make this a configuration setting, or include a button on-screen that the user can use to toggle the details for any TrackRefs. Ultimately, this could also be used to sort by artist or album on the fly.

The rules for displaying album details are currently as follows:

1. if we only have one track in the list, then show its album details
2. if we have more than one consecutive track from the same album, then add an album header above the tracks. Album details are not repeated for each of the album's tracks.
3. if we do not have any album info available or consecutive tracks are from different albums, then display a think grey divider between the tracks.

This PR also paves the way for fixing #69 and #96.

[jcass77]

Just a note that I have pushed some more changes which ensures that the loading popup is displayed until after all of the album details have been retrieved.

Also fixes an issue where the album details for the first track in the list was not being rendered.

[jcass77]

I've pushed a00754d which ensures that each track name is proceeded with an icon (except if part of a multi-track album) and a space character.

[jcass77]

jcass77@60041c1 refactors and consolidates how the 'Back' button is rendered at the top of track lists, and how messages are displayed when a folder does not contain any tracks.

jcass77@1c1fbee makes it more obvious if searches came up empty by showing 'No tracks found...' in the results table.

I'll run a few more tests and merge this tomorrow (Sunday) morning if there are no further inputs from anyone.

[kingosticks]

I'm away this weekend so I cannot try this.

[jcass77]

5437205 optimises the jQuery selectors to use identifiers instead of attributes. Lists with roughly 2 000 tracks are now rendered about 80 times faster.

Also fixed a small issue with play icons not being updated in the #albumstable, and replaced string references with their JavaScript constants.

[jcass77]

I've been using this for a few days without any apparent issues. I'll merge later tonight if there are no objections.