WIP: GNOME HIG Redesign

I've done a little work on a possible redesign for Evolution that brings it more in line with the GNOME Human Interface Guidelines. This redesign is not just about aesthetics but also about improving on the design principles, such as using to progressive disclosure to avoid overwhelming the user with unrelated and unactionable actions. Attached is a screenshot of the progress I've made.

I've switched to using a header bar with the goal of eliminating the menu bar and toolbar in order to maximize vertical content real estate. Since Evolution is supposed to be extensible, it's important that we accommodate multiple shell views. So I created a custom shell switcher widget that displays buttons for each shell view, pushing any button that doesn't fit into a popover box as seen in the screenshot.

In addition, I have been trying to get rid of deprecated features in GTK and use new features, such as GtkBuilder UI files, in order to reduce complexity. So far, this has resulted in 516 insertions and 1395 deletions for a net gain of 879 less lines.

Before I continue, I would like your input as maintainer of Evolution. I would hate to continue working in a disagreeable direction.