Revert unused caching part of a4ebaff

We did not use the caching of commands/urlhandlers; additionally, the
urlhandlers was called before password authentication.

Without the caching, we no longer need to wrap connect/disconnect.
Remove some MPD caching, causing troubles with password authentication

Merge remote-tracking branch 'others/hobophobe/remove-mpdhelper-caching2'
Rework all icons into the iconfactory

Icons require absolute paths, so sonata/ui/icons.glade gets read into a
string, the absolute path added to icons, and then the string builds
an icon factory.

Removed unused icons. Made icon naming consistent.
Fix mpdhelper ValueError on weird disc values

As with the fix that was present for the track property, disc could
be formatted as, eg, 1/2.  This fixes that, and moves the cleanup
for both track and disc into a common function.
Refactor song time texts

There are three:
1. About dialog
2. The main status bar
3. Library (in the item listings for genre, artist, and album views)

This updates the localization strings and should be more useful way to
handle for localizers.
Add more tests for weird disc values
Fix mpdhelper ValueError on weird disc values
Better translation support and formatting for time-related strings

Merge remote-tracking branch 'others/hobophobe/refactor_time_text'
i18n: the content of POTFILES.in is now stable (always sorted)
i18n: up-to-date French translation
Simplified and homogenized use of icons in Sonata

Merge remote-tracking branch 'others/hobophobe/iconfactory-from-string2'

Conflicts:
	sonata/library.py
Reindent Glade files using Glade

During the migration to GtkBuilder, most of the Glade files were edited
by hand instead of using Glade, since it was much more convenient for
the mass-changes that had to be done.
However, Glade doesn't really agree with the XML ordering, and used to
reorder everything, just upon saving an unmodified file (and removed
some "useless" properties).
I saved them again with Glade this time, fixed some properties which
disapeared; now, files content should be more stable when edited with
Glade.
covers: some albums exist on Last.fm without covers, deal with them
doc: revamped README file
Fix configuration loading when no config file available

Fixes #35
scrobbler: allow to scrobble again

This was like completely broken and probably untested since the move to
Python 3.
Deal with file names and paths using strings instead of bytes

GLib.filename_from_utf8() returns bytes which is not really expected by
Python's own path manipulation functions and is not practical actually
anyway.
This fixes some issues with songs containing non-ASCII characters, tested
using various locales.
doc: recommends PyGObject 3.7.4 or more

3.7.4 fixes the problem from #27. It only occurs if people have playlists
with non-ASCII characters, so most people are going to be safe with a earlier
release of PyGObject. This is the recommended version anyway.
doc: more informations for the PyGObject dependency
Library search: fix wrong parentheses

Use lower() on match string, not on matching result.