Grammar & style of i18n labels

Mostly very minor grammar and/or consistency fixes in
translatable labels, full sentences in help labels, etc.
Two label asignments are replaced with GTK+ stock items which
results in correct translations, standard mnemonics, optional
button icons, ... These two are `GO_FORWARD` and `APPLY`. The
latter replaces "Install" on the installation overview screen.
Use isoquery for countries list
Use isoquery for languages list
Use system supported locales for language selection screen
Use custom flags for Esperanto and Interlingua
Make language selection list sortable by columns

Some users of "obscure nationalities" may not know what their
language is called in English, yet they may more easily recognize
it by country or flag (when sorted). Can't hurt.
Remove invalid indentation in changelog (fixes minor warning from lintian)
Slideshow: remove reference to non-existing file
Merge pull request #45 from monsta/patch-5

Remove invalid indentation in changelog (fixes minor warning from lintian)
Merge pull request #46 from monsta/patch-6

Slideshow: remove reference to non-existing file
Merge pull request #44 from kernc/languages

Language selection screen refactoring
Merge pull request #34 from kernc/labels_fixes

Grammar & style of i18n labels
Show autologin option value in the overview
Merge pull request #47 from monsta/patch-7

Show autologin option value in the overview
Improve l10n a bit
Merge pull request #48 from monsta/patch-8

Improve l10n a bit
Debug printing on autologin setting change (for consistency)
Merge pull request #49 from monsta/patch-9

Debug printing on autologin setting change (for consistency)
Better debug and exception handling

With this commit:
* `--debug` switch now sets Python's `__debug__ = True` (and
  the lack of it enables `-OO` optimizations),
* ˙sys.excepthook˙ (default exception handler) is replaced by a
  function that first prints a normal traceback, and then issues
  Python debugger if `__debug__` is `True`, so the programmer can
  interactively inspect their error,
* a couple of really pointless try-except blocks,

  ```
  try:
      ...
  except Exception, e:
      print e
  ```
  have been unindented, with potential programmers' errors handled
  by said excepthook. Additionally, the unindented blocks hold the
  following changes:
  * stripped of all trailing whitespace,
  * gtk.gdk.threads_enter/leave() calls have been replaced by
    gtk.gdk.lock context guard.
Print locals() before entering pdb