Add vertical resizer between canvas

[kartikcho]

According to Brian's new mockup, we need to add a resizer in between the user timing graph and the flamegraphs.

Check Firefox profiler's implementation of the resizer between their two sections for direction and ideas.

We need to decide whether we should break the canvas into 2 or use 1 canvas. @kartikcho and Brian think splitting the canvasses will be easier, but @taneliang suspects the work done to achieve #50 may make it easy enough to implement the resizing in one canvas.

Depends on #30. Blocked by #50.

Approach: break canvas

Steps

• Breaking the existing canvas into two separate canvas.
• Inserting a resizer in between.

Acceptance Criteria

• Resizing must feel natural.
• There should be a different cursor on the resize handle.
• The resizer should not interfere with other mouse events.
• Scroll must remain in sync between the two canvasses.

Approach: one canvas

Steps

• Think of a way to implement this without changing the mouse event handling too significantly.
• Implement stacking of render sections within the canvas somehow (likely to be done in Fix performance issues likely caused by unnecessary renders #50).
• Implement a resizing mechanism.

Acceptance Criteria

• Resizing must feel natural.
• There should be a different cursor on the resize handle.
• The resizer should not interfere with other mouse events.