Add progress loading icon to QtQuick server row

Summary:
This adds loading icon in the same place where unread notification count
resides. These two will never interfer with each other because loading
is always finished when unread count is displayed.

Connection state enum had to be moved from namespace to Server class
because of QML support: QML only supports "nested" class enums or
namespace enums of new C++11 style (enum classes). Signals and typos
were transformed to new syntax and fixed along the way.

There are still some quirks that I'm unsure how to fix:

• Loading icon is pixelated when it rotates - strange given it's SVG
• Loading icon appears smoothly but disappears instantaneously because of QML destroy() behaviour.

Reviewers: hein

Reviewed By: hein

Differential Revision: https://phabricator.kde.org/D9708