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)
Allow installation to more than one drive

This commit somewhat overhauls the partition selection screen.
It introduces the following changes:

* parted binary is used for easy, optimal partitioning;
* the HDD selection screen is no more,
* instead, all attached drives (sans the rootfs one) are shown
  in the disks (partitions) treeview,
* and there is a button 'Expert mode' for the "custom mount onto
  /target instructions";
* all python code related to partitioning is in the new
  partitioning module;
* the colorful partitions webview is a standalone HTML file in
  usr/share;
* plus other related minor simplifications, additions & fixes.

Also closes #21.