Fix logic for left-aligned close button positioning

Note: This issue is not present in debug builds, and only appears in release builds (for reasons I can't begin to understand).

When using left-aligned close buttons, they currently display on the conversation header bar, in the middle of the window:

This is frankly pretty bizarre, and doesn't match behavior for other applications, such as Gnome Settings, which correctly displays the close buttons on the left-hand side:

This pull request fixes the close button positioning so that the buttons properly display on the left-hand side of the main headerbar when they're left-positioned.