GH-421: Cleaned up the _Output_ channel UI.

- Merged the Arduino channels into one,
 - Removed the channel selector dropdown from the UI.

Closes arduino/arduino-pro-ide#421.

Signed-off-by: Akos Kitta <kittaakos@typefox.io>

arduino-ide-extension/src/browser/theia/output/output-toolbar-contribution.ts

@@ -0,0 +1,15 @@
+import { injectable } from 'inversify';
+import { ReactTabBarToolbarItem, TabBarToolbarItem, TabBarToolbarRegistry } from '@theia/core/lib/browser/shell/tab-bar-toolbar';
+import { OutputToolbarContribution as TheiaOutputToolbarContribution } from '@theia/output/lib/browser/output-toolbar-contribution';
+
+@injectable()
+export class OutputToolbarContribution extends TheiaOutputToolbarContribution {
+
+    async registerToolbarItems(registry: TabBarToolbarRegistry): Promise<void> {
+        await super.registerToolbarItems(registry); // Why is it async?
+        // It's a hack. Currently, it's not possible to unregister a toolbar contribution via API.
+        ((registry as any).items as Map<string, TabBarToolbarItem | ReactTabBarToolbarItem>).delete('channels');
+        (registry as any).fireOnDidChange();
+    }
+
+}