Add margin to left edge of New/Edit annotation panel

[ggeisler]

I know the New/Edit annotation panel is only in a first draft version (there are more features proposed in #4) but we could make it feel a little more polished and friendly to users with a minor styling update.

Currently there is no whitespace between the left edge of the panel and the left-edge of the panel title and panel content elements:

I think we're using padding-left: 16px; for the left edge margin on the default window sidebar; I suggest using the same value for the New/Edit annotation panel.