User notes / bookmarks support

[cas4ey]

Original message from @rationalcoder (see #91):

At my work, I am trying to enforce the practice of providing before-and-after profiling results of patches that claim to have made something faster, along with providing profiling results when claiming that something is slow in the first place. This is currently annoying, as a capture might be 10+ seconds long, with only a small portion of it containing an example of the issue.

I would like the ability to select a region and be able to export only it into a smaller .prof file. In addition, I would like to be able to add notes at certain time-stamps. That would be useful if, for example, there are two or more frame spikes in question that are dependent on each other in some way.

Snapshots should contain the relevant FPS and user-defined metrics that are introduced in the future, so that notes can reference that data. In addition, snapshots should contain enough information for the GUI to reproduce at least the zoom level / vertical scroll % (to take you right to a problematic thread).

I imagine this being optional data defined to be stored after the usual contents of a .prof. That way, any additional overhead is only on the GUI side, where it doesn't matter.

The snapshot feature is implemented. This issue is about adding user notes / bookmarks to the scene.

[cas4ey]

Use double-click on the timeline in Diagram widget to add/edit/remove bookmarks.
Bookmarks can be dragged over the timeline with left mouse button.
File needs to be saved after adding/editing/removing bookmarks.