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.schemesURI=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.

Events are routed to a new app.Events, which are not linked to a specific window.

Signed-off-by: inkeliz <inkeliz@inkeliz.com>
Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/GioActivity.java

@@ -4,6 +4,7 @@ package org.gioui;
 
 import android.app.Activity;
 import android.os.Bundle;
+import android.content.Intent;
 import android.content.res.Configuration;
 import android.view.ViewGroup;
 import android.view.View;
@@ -29,6 +30,7 @@ public final class GioActivity extends Activity {
 
             layer.addView(view);
             setContentView(layer);
+            onNewIntent(this.getIntent());
 	}
 
 	@Override public void onDestroy() {
@@ -60,4 +62,9 @@ public final class GioActivity extends Activity {
 		if (!view.backPressed())
 			super.onBackPressed();
 	}
+
+	@Override protected void onNewIntent(Intent intent) {
+		super.onNewIntent(intent);
+		view.onIntentEvent(intent);
+	}
 }