Clear empty fields in MP4 tags

The existing tag is loaded from the file and modified before being
written out. For "extra" fields, including cover art and album artist,
the existing tag field was preserved in the case that the corresponding
tag field had been cleared from the UI. Fix this by explicitly removing
the cleared tag field.
Use level 2 interface when reading FLAC header

Improve bitrate calculation to ignore metadata blocks.
Add a setting to disable automatic image types

The heuristic for selecting the type of an image based on the filename
can get confused quite easily, so provide an option to disable it.

https://bugzilla.gnome.org/show_bug.cgi?id=738804
Refactor Vorbis comment setting functions slightly

Iterate over string array better, without using g_strv_length(). Make
more parameters const. Simplify conditional check. Remove a couple of
unnecessary prototypes.
Fix GIOStream leak when checking for buggy id3lib
Updated Polish translation
Remove unused filename parameters in header code

Avoid some -Wextra warnings about unused parameters.
Avoid GSettings handler unused parameter warnings

Use the settings and key parameters to the GSettings changed handler
where possible.
Use const in more places
Remove unused et_grid_attach_full()
Use GFile in header reading functions
Use GFile for tag reading functions
Remove unused log area show/hide functions
Move log area visible logic to EtApplicationWindow
Remove outdated configuration file handling
Fix settings key name for default path
Fix CDDB search button tooltip text
Do not show the PID of the application at startup

Any warning message from g_log() will show the PID, and it is otherwise
not that useful.
Check for gdk-pixbuf-pixdata in configure

The gdk-pixbuf-pixdata tool is required for glib-compile-resources when
using the to-pixdata option.
Fix a MinGW build failure in EtBrowser