Warnings are assigned a category.

Warnings can be filtered by category, by message regex, by site where
they are issued, and by source path. Deprecations can additionally be
filtered by origin (deprecated definition) and `since` version.

Filtered warnings can be reported as error, warning, info, summary
(like deprecations) or silent.

Adds a `-rootdir` compiler flag. It is used to relativize file paths
when using source filters (`-Wconf:src=some/source/File.scala:s`).
There might be other uses for it in the future.

Unchecked warnings are now all shown by default (they were summarized
like deprecations before).

The `-deprecation`, `-feature` and `-unchecked` settings are no longer
directly used in the compiler, they are shortcuts for specific `Wconf`
configurations. The compiler only looks at `-Wconf`.

Message filtering is performed in `global.currentRun.reporting`, not
in the `Global.reporter`. The reasons are:
  - Separation of concerns: `Reporter`s worry about how to do reporting,
    removing duplicate messages.
  - Custom `Reporter`s are used by sbt, silencer, REPL, etc. It's too
    hard to do the necessary changes to the `Reporter` interface.
  - The `Wconf` setting could change between compiler runs.
    `currentRun.reporting` respects those changes.

So all warnings in the compiler should go through `global.runReporting`
(which is the same as `global.currentRun.reporting`). This method takes
four parameters: pos, msg, category (new), site (new). The site is
usually `context.owner` (in the frontend) or `currentOwner` (in
transformations).

`Context` has a `warn` method with 3 parameters (pos, msg, category) and
inserts the `owner` as `site`, so this is used in the frontend (context
reporters are also used to support silent mode / trying twice).

The `global.warning` method is deprecated and no longer used.

There are a few calls to `Reporter.warning` left in the codebase where
no `runReporting` is reachable, I think they are all OK not to
categorize / allow filtering. E.g., when running Scaladoc

```
reporter.warning(null, "Plugins are not available when using Scaladoc")
```

Integrate the fantastic [silencer](https://github.com/ghik/silencer)
compiler plugin by @ghik into the compiler, which allows suppressing
warnings locally using he `@nowarn` annotation.

The `@nowarn` annotation suppresses warnings within the scope covered by
the annotation.
  - `@nowarn def foo = ...`, `@nowarn class C { ... }` suppress
    warnings in a definition
  - `expression: @nowarn` suppress warnings in a specific expression

The annotation can be configured to filter selected warnings, for
example `@nowarn("cat=deprecation")` only suppresses deprecation
warnings. The filter configuration syntax is the same as in `-Wconf`.

MiMa exception for addition of `scala.annotation.nowarn`

Reporting warnings is now delayed until after typer, because the typer
collects `@nowarn` annotations.

If we stop before typer (e.g., because there are errors in the parser),
all warnings are shown, even if they are enclosed in `@silent`.

If a phase before typer has both errors and warnings, all errors are
printed before the warnings.