Update tutorial URLs to point to new docs site

Fix bug 3097
Make the main.py file directly runnable again
Improve rendering on Windows 7

There is no backgrond colour for the source text, and this helps to
frame it clearly and helps to see different suggestions more easily.
Unify API for native file dialogs
Specify the handle on win32

This way the dialog knows its parent and the main window can't be
selected.
Add newer Windows versions
Add systemd release info

This fixes bug 2105.
Don't cache alt trans and limit emit calls

Now we cache empty results to avoid repeated (useless) queries. We
also stop caching alt trans results, which depend on more than just
the source text (this is unique per unit).
Only evict units affected by a change in translation

Only certain units in the cache can possibly be affected by a new
translation, so we don't need to obliterate the whole cache.
Cosmetic
Factor out common code
Updated the translation

According to Friedel's comment, thank you man! It was very nice meeting you ☺
Remove unused imports
Cosmetic comment updates
Install a fake gettext function if setlocale() failed
Use local variable that is available
Fix typo.
Ignore stuff from the 'gh-pages' branch.
Remove unused builder filename
Cache builders to avoid unnecessary reparsing