* io,app: add custom scheme support

Now, it's possible to launch one Gio app using a custom URI scheme, such as `gio://some/data`.

This feature is supported on Android, iOS, macOS and Windows, issuing a new transfer.URLEvent,
containing the URL launched. If the program is already open, one transfer.URLEvent will be
sent to the current  app.

Limitations:
On Windows, if the program listen to schemes (compiled with `-schemes`), then just a single
instance of the app can be open. In other words, just a single `myprogram.exe` can
be active.

Security:
Deeplinking have the same level of security of clipboard. Any other software can send such
information and read the content, without any restriction. That should not be used to transfer
sensible data, and can't be fully trusted.

Setup/Compiling:
In order to set the custom scheme, you need to use the new `-schemes` flag in `gogio`, using
as `-schemes gio` will listen to `gio://`.

If you are not using gogio you need to defined some values, which varies for each OS:

macOS/iOS - You need to define the following Properly List:
```
<key>CFBundleURLTypes</key>
<array>
  <dict>
        <key>CFBundleURLSchemes</key>
        <array>
          <string>yourCustomScheme</string>
        </array>
  </dict>
</array>
```

Windows - You need to compiling using -X argument:
```
-ldflags="-X "gioui.org/app.schemesDeeplink=yourCustomScheme" -H=windowsgui"
```

Android - You need to add IntentFilter in GioActivity:
```
<intent-filter>
        <action android:name="android.intent.action.VIEW"></action>
        <category android:name="android.intent.category.DEFAULT"></category>
        <category android:name="android.intent.category.BROWSABLE"></category>
        <data android:scheme="yourCustomScheme"></data>
</intent-filter>
```

That assumes that you still using GioActivity and GioAppDelegate, otherwise more
changes are required.

Signed-off-by: inkeliz <[email protected]>

* app: [ios/macos] remove mutex

Signed-off-by: inkeliz <[email protected]>

* io: fix documentation

Signed-off-by: inkeliz <[email protected]>

* app: [windows] change name and value of the scheme message identifier

Signed-off-by: inkeliz <[email protected]>

---------

Signed-off-by: inkeliz <[email protected]>

This patch is intended to fix two issues related to focus and
software-keyboard.

The focus is now reseted if it's lost due to external event,
such as clicking on non-Gio window/activity/fragment.

The software-keyboard will now re-open when clicked, except
if it's ReadOnly.

On macOS it will request focus again if the focus is lose
due to another View in the same Window. However, currently,
it's done by using ShowTextInput.

Fixes: https://todo.sr.ht/~eliasnaur/gio/591

Signed-off-by: inkeliz <[email protected]>

Previously, setHighRefreshRate requires APIs restricted
to Android 30, or higher.

Tested on Android 6.0.1 (released on 2015).