Initial JetBrains support - Idea and PyCharm Community Editions.

Work in progress, do not merge yet.

DownloadCenter grew support for custom headers in DownloadItems.
Support for PyCharm and Idea Community Editions.

Need to add tests and refactor the JetBrains stuff a little so more logic is in the base JetBrains installer - this will allow us to add support for more IDEs easily.
Do you mind adding it to the build-depends as well? That would be useful to stay in sync the day we eventually run in adt the medium or large tests.

Same, even if I'm sure it's not up to date, adding it to requirements.txt will help the day we gain some interests into the virtualenv again (if the gobject-introspection state on pipy gets better)