This PR was squashed before being merged into the 4.3-dev branch (closes #30450).

Discussion
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[Profiler] Render the performance graph with SVG

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | Part 1, 3 and 4 of #27262
| License       | MIT
| Doc PR        | n/a

Following a suggestion by @ogizanagi in #27262,
Here's a proposal to render the Request Graph, from the performance profiler panel, with SVG instead of canvas.

Some benefits of the SVG format:
- The text labels are searchable and can be selected.
- It renders well on high DPI monitors.
- [Colors and text styles](#27262 (comment)) can be defined with CSS just like the rest of the page.

In addition, SVG allow us to consider (and easily implement) interactives features such as:
- Zoom in and time navigation (thanks to the viewport).
- Highlight hovered line (or other DOM related events).

Preview:

![screenshot_2019-03-08 symfony profiler 1](https://user-images.githubusercontent.com/1846873/54036727-a33f4300-41bc-11e9-8be7-b1de10d4afd9.png)

Filtered events example:

![capture d ecran 2019-03-08 a 17 22 47](https://user-images.githubusercontent.com/1846873/54041039-00d88d00-41c7-11e9-9590-23e809415c34.png)

### Progress :
- [x] Render request events in SVG
- [x] Show labels with duration and memory
- [x] Show specific markers at start / end of lines
- [x] Re-render graph when window resize
- [x] Re-render graph when threshold change.
- [x]  Generate graph legend with only existing categories (part 1. of #27262 )
- [x] Show sub-request area with hatched pattern
- [x]  Allow to hide categories by clicking them on the legend (part 3. of #27262 )
- [x] Handle text overflow on long labels.
- [x] Ensure JS code is compatible with all supported browsers (used [classes](https://caniuse.com/#feat=es6-class) and [arrow functions](https://caniuse.com/#feat=arrow-functions).
- ~Add left-padding to sub-request graph?~

Commits
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a69a718 [Profiler] Render the performance graph with SVG

This PR was merged into the 4.3-dev branch.

Discussion
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[Profiler] Fix graph text color

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | yes
| New feature?  | no <!-- don't forget to update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- don't forget to update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | -   <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | -

#30450 introduces bad text color for graph items in the profiler, specially in dark theme mode.

### before/after

![Capture d’écran de 2019-04-06 21-38-29](https://user-images.githubusercontent.com/7721219/55674505-ff79ad80-58b5-11e9-9a29-8359ff07673d.png)

![Capture d’écran de 2019-04-06 21-42-26](https://user-images.githubusercontent.com/7721219/55674507-030d3480-58b6-11e9-8dee-395e2046e7ca.png)

There is a small change for default theme

### before/after

![Capture d’écran de 2019-04-06 21-51-46](https://user-images.githubusercontent.com/7721219/55674538-4ff10b00-58b6-11e9-8aed-2621bdde673d.png)

![Capture d’écran de 2019-04-06 21-52-15](https://user-images.githubusercontent.com/7721219/55674540-52ebfb80-58b6-11e9-9d94-68743187aba2.png)

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Commits
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44d9fbe Fix graph text color