feat: shorten thread names

The Linux kernel imposes a 15-byte limit on thread names[1]. This means
information that does not fit in this limit, e.g., the pipeline ID of
layout and script threads, is lost in a debugger and profiler (see the
first column of the table below).

This commit shortens the thread names used in Servo to maximize the
amount of information conveyed. It also rectifies some inconsistencies
in the names.

|       Before      |       After       |
|-------------------|-------------------|
| `BluetoothThread` | `Bluetooth`       |
| `CanvasThread`    | `Canvas`          |
| `display alert d` | `AlertDialog`     |
| `FontCacheThread` | `FontCache`       |
| `GLPlayerThread`  | `GLPlayer`        |
| `HTML Parser`     | `Parse:www.examp` |
| `LayoutThread Pi` | `Layout(1,1)`     |
| `Memory profiler` | `MemoryProfiler`  |
| `Memory profiler` | `MemoryProfTimer` |
| `OfflineAudioCon` | `OfflineACResolv` |
| `PullTimelineMar` | `PullTimelineDat` |
| `ScriptThread Pi` | `Script(1,1)`     |
| `WebWorker for h` | `WW:www.example.` |
| `ServiceWorker f` | `SW:www.example.` |
| `ServiceWorkerMa` | `SvcWorkerManage` |
| `Time profiler t` | `TimeProfTimer`   |
| `Time profiler`   | `TimeProfiler`    |
| `WebGL thread`    | `WebGL`           |
| `Choose a device` | `DevicePicker`    |
| `Pick a file`     | `FilePicker`      |
| `Pick files`      | `FilePicker`      |

[1]: https://stackoverflow.com/questions/5026531/thread-name-longer-than-15-chars

components/canvas/webgl_thread.rs

@@ -312,7 +312,7 @@ impl WebGLThread {
     /// in parallel on its own dedicated thread.
     pub(crate) fn run_on_own_thread(init: WebGLThreadInit) {
         thread::Builder::new()
-            .name("WebGL thread".to_owned())
+            .name("WebGL".to_owned())
             .spawn(move || {
                 let mut data = WebGLThread::new(init);
                 data.process();