Pythons urllib supports proxys via env vars `http_proxy` and
`https_proxy`.

If someday needed this should be reimplemented with urllibs ProxyHandler

Fixes #262

[chatstate] fix color converting from gtk floats

See merge request gajim/gajim-plugins!59

Updated old Urls in code and manifest.

OMEMO Make some more strings translatable

See merge request gajim/gajim-plugins!57

[preview] Display preview beginning with a new line and add tooltip

See merge request gajim/gajim-plugins!56

available

Even if we dont need account for omemo, its easier to supply always
the same count of arguments to all encrypt_file() plugin methods.

[OMEMO] Make some strings translatable

See merge request gajim/gajim-plugins!54

Now we dont need to supply the account