Better error messages when encoder cannot be created

The vorbis encoder cannot be initialized for very low bitrates. In
these cases a meaningful error message is printed to the console.
This is still not the best solution, but better than nothing.

git-svn-id: https://svn.code.sf.net/p/aqualung/code/trunk@1291 3cd24cdc-1f22-0410-b8b1-dcf80e670293
Merge branch 'b141_not_so_modal_dialogs'

git-svn-id: https://svn.code.sf.net/p/aqualung/code/trunk@1292 3cd24cdc-1f22-0410-b8b1-dcf80e670293
Refactor file info dialog invocation and access to tree stores

This is needed to create a feature allowing to go to the previous /
next file in the natural order of wherever the file info dialog was
opened from, without closing and reopening the dialog.

The prev/next buttons to do this are already on the file info dialog,
but they don't yet rebuild the dialog content. However, the generic
tree traversal is already implemented, taking care that the containing
store (in case the dialog was opened from a file or podcast store)
cannot be left while browsing.

git-svn-id: https://svn.code.sf.net/p/aqualung/code/trunk@1293 3cd24cdc-1f22-0410-b8b1-dcf80e670293
Refactor file_info using so-called subroutines

git-svn-id: https://svn.code.sf.net/p/aqualung/code/trunk@1294 3cd24cdc-1f22-0410-b8b1-dcf80e670293
Implement file info dialog's prev/next track feature.

The newly added buttons at the bottom right of the dialog now do their
thing: the whole dialog is refreshed with data from the previous/next
track (according to the natural ordering of the tree model from where
the dialog has been opened from).

Special care has been taken to implement the following corner cases:
- files not accessible anymore
- alternation of read-only/writable files
- files with or without cover images besides them
- files with or without embedded image metadata
- do not lose dirty (edited) metadata
- if possible, stay on the same tab for the new file

Quite an amount of refactoring was done along the way, hopefully for
the better.

This puts a glorious end to bug#158 [next file & prev file in File
Info gui]

git-svn-id: https://svn.code.sf.net/p/aqualung/code/trunk@1295 3cd24cdc-1f22-0410-b8b1-dcf80e670293
Fix: file info dialog across CDDA and disk tracks

git-svn-id: https://svn.code.sf.net/p/aqualung/code/trunk@1296 3cd24cdc-1f22-0410-b8b1-dcf80e670293
Fix compilation in the presence of the WavPack decoder library

git-svn-id: https://svn.code.sf.net/p/aqualung/code/trunk@1297 3cd24cdc-1f22-0410-b8b1-dcf80e670293