Fixed some errors while loading a file.
Fixes and improvements in tabs opening.

* Double clicking on sidepanel list now opens already existing tab instead of a new one.
* Opening a new file to edit now brings focus to it (changes tab to a proper one when while opening
  it)
* Changing tabs somehow started to work. Maybe it was just a broken test procedure? (just use
  totally different subtitles instead of similar ones)
* There are some TODOs and FIXMEs added. Should ConverterManager be removed?
Fixes and improvements

* SubConverterManager and SubConverter interfaces made uniform.
* File list tooltip text changed to a file path.
* Added some "FIXME" comments in places that need further work.
* Modified test_converting so it fits a current interface.
Added Qt resources file to handle images.

A resources.py module MUST be generated before execution or after every resources.qrc change.
A command used to generate it:
    pyrcc4 -py3 -o resources.py ./resources.qrc
Added generated resources module.
Removed file path dependency from SubConverter

* From now on SubConverter and parsers must not log events. Use exceptions or some other mechanism
  instead.
* Information about file path is now only available through SubConverterManager. It's also the only
  interface to choose between several SubConverters. Differenting SubConverters via so-called
  'aliases' (which should be registered to a filename) will be implemented later.
* GUI SubConverterManager instance has been moved from SidePanel to SubTabWidget which is the main
  class/container which stores and has direct access to other GUI elements.
* In the next step SubConverter should be covered with tests so further refactoring will be
  possible.
* Refactoring should also cover better header handling. It's not a good design where it's mixed (or
  not) with a first subtitle. It should be stored independently to subtitles, possibly in
  SubConverter class.
Added 'alias' to SubConverterManager

SubConverter might be accessed via so-called 'alias'. It's unique string mapped to a particular
file path (i.e. a single file path might have several aliases but each alias might point to only one
path). SubConverterManager might register or deregister any alias at any time. When it's done, every
access to SubConverter can be done either via alias or a filename.