Make the file list treeview columns resizeable

Fixes bug 694312.
Refactor picture type from filename heuristic

At the suggestion of Oliver Joos, change the default type when adding a
picture to "front cover":

https://mail.gnome.org/archives/easytag-list/2013-March/msg00015.html

Refactor the picture type heuristic into a separate function.
Make the picture type mapping array static
Updated Slovenian translation
Improve TagLib error handling, bug 695453

Avoid dereferencing NULL pointers returned from TagLib when an MP4 file
could not be read.
Use Log_Print rather than g_print in ID3v2 reader
Updated Hungarian translation
Updated Lithuanian translation
Updated Polish translation
Updated Serbian translation
Updated Slovenian translation
Updated Slovenian translation
Use "image" instead of "picture" in the UI

Replace all instances of "picture" with "image" in the UI.

Remove the redundant "Pictures:" label next to the images tree view,
move the "apply to all" button below the tree view, alongside the other
buttons, and give it an insert-image icon.
Remove the frames from the tag and image tabs

The shadow type was set to not show the frame.
Use a toolbar for the picture tab buttons

Use a toolbar for the control buttons underneath the picture tab tree
view. Update the sensitivity of the toolbar buttons depending on the
selected items (make the remove, save and property buttons insensitive
if no items are selected). Improve the string for removing images.
Remove some commented-out code for setting the tag area widgets to be
insensitive.
Slightly improve size conversion functions

Use lower-case 'k' for kilo prefix. Use gsize for sizes. Use off_t when
getting the file size from stat().
Use the Unicode multiplication sign for dimensions
Updated Polish translation
Remove ancient libmpg123 code, bug 692410

Use id3lib for reading the MPEG header instead of the internal version
of mpg123. With only id3tag support (and no id3lib support) the MPEG
header is not read, but ID3v2.4 tag reading and writing still works.
Modify ifdefs and update .gitignore appropriately for libmpg123 removal.
Move ID3_INVALID_GENRE to src/id3_tag.h to avoid a warning.
Make the sort options more complete

Some of the code has been reordered to match the order in the enum of
sort modes. Also, the sorting function for the 'Disc Number' tag has
been added. Partially fixes bug 697570.