Settings pane

This commit contains a settings pane implementation. There are a couple
of things here, which are not used yet, like advanced form helpers. I'm
leaving them here, because the plan is to add support for more settings
soon (like: include/exclude branch patterns), which will need these
helpers.

There is also tabs support, although in the current version there is
only one tab (initially it was created for supporting general tab and
notifications tab).