Add support for markers

[dominikh]

Allow users to set markers in time. These markers will show up on the axis as some kind of icon, and on the canvas in a way similar to the vertical line of the cursor. Markers will have colors. Right-clicking on the axis will offer the option to create a marker, which opens a dialog window, allowing to adjust the timestamp, specify a name, and specify a color. There will also be new shortcuts and dialogs to list markers by name/time and to jump to them.

Other places that might want to offer the option to create markers: when right-clicking a timestamp in the UI, when right-clicking a span (which should offer creating a marker at its beginning or end).

/cc @whereswaldon

[whereswaldon]

Yeah, timestamps and/or events being markable would be awesome.