filesystem-qt: Introduce new plugin to manage local audio files

[radioactiveman]

It basically serves as an embedded file manager view

• allowing to create or extend playlists
• opening the music folders and cover art
• supporting drag and drop

Depends on the new libaudcore function added in audacious-media-player/audacious#1671.

Supersedes PR #184 by @hdijkema with a simpler and more straightforward approach.

[jlindgren90]

This is very handy, nice job. You can merge it as-is if you want - below are just some ideas/suggestions.

Naming

• How about naming the plugin just "File Browser"? That seems more to the point than "Filesystem Manager".
• Consider renaming "Open Folder" to "Open Folder Externally" to be more clear what it does.

UI

Consider removing the "Quick Search" toggle and making the Search bar always visible. It doesn't take much space and is a very important part of the plugin IMO.

The choice between "Tree View Mode" and non-"Tree View Mode" feels awkward and unnecessary. The only advantage that I can see of the non-"Tree View Mode" is that it's much easier to navigate to a different base folder (which also makes the separate "Change Music Folder" dialog redundant).

What about making "Tree View Mode" the only mode, with the following improvements?

• Add a small toolbar, with an "up" button (to navigate up one level) + a label with the name of the current base folder.
• In the context menu, add an option "Set Base Folder" which would immediately navigate to the selected folder.
• Then the "Change Music Folder" dialog would be unnecessary, and could be removed.

Edit: or, to save space, combine the toolbar and search bar. "Up" button to the left of the search input, with the placeholder text saying "Search in (base folder name)".

[jlindgren90]

Quick-and-dirty implementation of above ideas (except renaming the plugin):
https://github.com/jlindgren90/audacious-plugins/commits/filebrowser/

[jlindgren90]

Some issues I've noticed in a quick test:

• The quick search only finds items in the base folder and doesn't seem to be recursive at all (even into folders that are visually expanded). So it's not actually that useful.
• Sometimes when expanding a folder, multiple rows get expanded. It seems random; I can't reproduce it consistently. Could be a Qt bug.

Edit: one more. I would expect to be able to play a file in the list by double-click. This doesn't appear to work.

[jlindgren90]

jlindgren90@af06b11 gets recursive search mostly working. It can still be slow and/or unreliable though. It seems QFileSystemModel is not really built for robust file search capability.