[PICARD-519] Fix up hacky resize/restore/save of metadata box columns

Replace `metadata_box_sizes` option by QByteArray `metadatabox_header_state` and
simply rely on Qt `QHeaderView::restoreState()` and `QHeaderView::saveState()`

This change implies metadata box columns aren't auto resized when changing main window size, but instead scrollbar may appear.

Overall, it is still user-friendly enough and much more reliable.

See http://tickets.musicbrainz.org/browse/PICARD-519
Fix column widths to fit header labels on first start
Merge pull request #351 from zas/metadatabox_sizes

[PICARD-519] Fix up hacky resize/restore/save of metadata box columns
Make columns of equal sizes at first start
Merge pull request #353 from zas/metadatabox_sizes_2

Metadata box: make columns of equal sizes at first start
Display a simple dialog to select cover art types

It is much more convenient than the list widget.
The dialog includes "Check All" and "Uncheck All" buttons, as well a "Help" button.
Tidy up
Fix conflicting accelerator

"Cancel" button is already using "C", so use "k" for "Check All"
Add option to disable all restrictions to CAA types

This is better since the list of types may change, adding a "do not restrict" ensure
all types are downloaded if more types are added to MB.

If restriction is enabled, then previous behavior is used (downloading only images having specified types).
Set "caa_restrict_image_types" to True by default.

This way old behavior is preserved, restricting to defined types.