Explorer window header

[martinmcclure]

Explorer windows need a header bar that goes the entire width of the window, just under the title bar. The header bar should have:

• A background color that is specified by the connection parameters used to create its connection. If connected to more than one stone at once, this will help the user identify windows.
• At the left, text describing the connection parameters, as additional identification.
• At the right, a hamburger menu, initially showing undo and redo (possibly more operations here in the future). The menu may not fully work right away due to #33, but key bindings (ctrl-Z and ctrl-shift-Z) with a scope of the entire window should be established.

[martinmcclure]

We've added a hamburger menu to the window title bar, a partial solution to this issue. It's possible the other points could also be solved within the window title bar rather than a separate header bar. For color, could do either a color patch in the title bar or color the entire window frame. Connection name could replace or add to the existing generic "Sparkle Explorer" label.