Plugin api window.activeTerminal can't get the right active terminal

[godlin-gh]

Description

I've written a vscode extension(also a theia plugin) which use the api window.activeTerminal. It works well on vscode but not on theia.

I've debugged the theia source code and found the bug here:

theia/packages/terminal/src/browser/terminal-frontend-contribution.ts

Lines 116 to 120 in c99cc1d

	protected readonly onDidCreateTerminalEmitter = new Emitter<TerminalWidget>();
	readonly onDidCreateTerminal: Event<TerminalWidget> = this.onDidCreateTerminalEmitter.event;
	protected readonly onDidChangeCurrentTerminalEmitter = new Emitter<TerminalWidget | undefined>();
	readonly onDidChangeCurrentTerminal: Event<TerminalWidget | undefined> = this.onDidCreateTerminalEmitter.event;

The onDidChangeCurrentTerminal should not be assigned this.onDidCreateTerminalEmitter.event, it shoud be this.onDidChangeCurrentTerminalEmitter.event

Reproduction Steps

1. debug source code
2. set breakpoint on TerminalServiceMainImpl.updateCurrentTerminal method
3. change active terminal on theia UI, you'll find out that the breakpoint never be reached.

or you can try to write a plugin use window.activeTerminal api

OS and Theia version:
master
Diagnostics:

[akosyakov]

@Godlinbupt please feel free to send a PR

[godlin-gh]

@Godlinbupt please feel free to send a PR

OK，I will send a PR later.