Various UI improvements

1. Add consistent set of StatusTips / ToolTips
2. Move Refresh from View to Tools (its an action not changing view)
3. Rename and resequence Help menu.
4. Make sequence of files in a directory in Unmatched files be alphabetical (which should match album track sequence)
5. Make History text more consistent and user friendly and add Remove Album message.
6. Add Open Collection in Browser and reload collection if user details are changed in options
Swap Add Files/Directory...

... to reflect likely priority of usage i.e. most people will load
directories rather than files.
Address zas comments
Address Bitmap comments.
Address more Bitmap comments
Address even more Bitmap comments

Keep them coming Michael - every review comment = an improvement.
Fix tab order which I failed to adjust.
And some more Bitmap comments...

which didn't show on the GitHub comments page.
Add comment for zas
Revert new status tips / tooltips ...

... and tweak browser integration tooltip.
Address various zas comments
Make Lookup CD / Browser consistent, ...

... and fix if statement.

(zas comments).
Add Play File plus icons ...
Fix makeqrc same results Windows and Mac
Merge pull request #238 from Sophist-UK/sophist_add_open_collection_in_browser

Various UI improvements
Update pot file
Add `update_constants` command to setup.py

It retrieves and parses attributes.pot and countries.pot files
from transifex to extract various constants needed in Picard.
It helps to keep those in sync with MusicBrainz website.

It then generates:
- picard/attributes.py containing cover art types, medium formats, and release group types
- picard/countries.py containing countries

`update_constants` command replaces `update_countries`
Update picard/attributes.py and picard/countries.py

New setup.py update_constants command was used.
Add ugettext_attr() function needed to translate attributes

Since contexts are used in attributes.pot, and python gettext do not yet
support contexts, a minor hack is used to make it work, this workaround
is used in django and babel, see ugettext_ctxt().
Build RELEASE_FORMATS and CAA_TYPES from MB_ATTRIBUTES