gtk3: CSS's font-size needs an unit, assuming pixel

Fix a warning while launching Sonata
Revert "gtk3: CSS's font-size needs an unit, assuming pixel"

This reverts commit 293be58bfb7455d8b1ca91150e9f4a912e6ab124.

It seems that older versions of Gtk don't support setting an unit here,
which makes Sonata crashes.
I have no idea from which version this support has been added, nor when
it started to get deprecated, so I'm just going to forget this warning
for now.

Fix #40
current: centering the song on the current playlist also selects the song

This occurs at startup and when using the Ctrl+i shortcut, which allows
for faster playlist manipulation.
i18n: update translation files and set up a Transifex project

Sonata is now translatable at:
https://www.transifex.com/projects/p/sonata/
i18n: fix broken translation files so they can be uploaded to Transiflex
i18N: add a README information about translation
i18n: now use Transifex as a translation plateform

Merge branch 'i18n-transifex'
doc: updated CHANGELOG with Transifex
i18n: update uk translation
i18n: update ru translation
doc: update CHANGELOG with new translations
tabs: correctly save current tabs visibility flag

Fix: #45
tray: better name for callbacks
tray: remove duplicated 'activate' signal handler

For the sake of convenience, everything is handled by the
'button-press-event' handler, since we are dealing with several mouse
buttons in the application.

Previously, left-clicking on the tray icon used to trigger both the
'button-press-event' and the 'activate' event, which actually tried to do
the same thing.
tray: simplify logic to decide either to hide or show the main window
tray: fix hiding/showing the main window even if it's not the active widget

If the main window is not the active widget (ie. if it doesn't have the
focus), it can now be hidden/shown by left-clicking on the tray icon.

Fix #43
tray: fix hiding/showing the main window even if it's not the active widget

Merge branch 'fix/43-show-hide-click-tray-window-not-active'
stream: fix streams parsing since Python 3 port

Inspired by Anders Boesen Lindbo Larsen patch from
https://github.com/multani/sonata/pull/41

Fix #41
Fix #34
stream: improve code readibility of stream parsing, simplify logic
stream: renaming