Add common top-level Action type

This makes for better type inference.

src/components/FlashButton.tsx

@@ -2,7 +2,7 @@
 // Copyright (c) 2020 The Pybricks Authors
 
 import { connect } from 'react-redux';
-import { Dispatch } from 'redux';
+import { Dispatch } from '../actions';
 import { flashFirmware } from '../actions/bootloader';
 import * as notification from '../actions/notification';
 import { RootState } from '../reducers';

src/components/LoadButton.tsx

@@ -2,7 +2,7 @@
 // Copyright (c) 2020 The Pybricks Authors
 
 import { connect } from 'react-redux';
-import { Dispatch } from 'redux';
+import { Dispatch } from '../actions';
 import * as editor from '../actions/editor';
 import * as notification from '../actions/notification';
 import { RootState } from '../reducers';

src/components/Notification.tsx

@@ -4,7 +4,7 @@
 import React from 'react';
 import Toast from 'react-bootstrap/Toast';
 import { connect } from 'react-redux';
-import { Dispatch } from 'redux';
+import { Dispatch } from '../actions';
 import * as notification from '../actions/notification';
 
 interface DispatchProps {

src/components/SaveButton.tsx

@@ -2,7 +2,7 @@
 // Copyright (c) 2020 The Pybricks Authors
 
 import { connect } from 'react-redux';
-import { Dispatch } from 'redux';
+import { Dispatch } from '../actions';
 import * as editor from '../actions/editor';
 import { RootState } from '../reducers';
 import ActionButton, { ActionButtonProps } from './ActionButton';

src/components/Terminal.tsx

@@ -4,10 +4,10 @@
 import React from 'react';
 import { connect } from 'react-redux';
 import ResizeObserver from 'react-resize-observer';
-import { Dispatch } from 'redux';
 import { Subscription } from 'rxjs';
 import { Terminal as XTerm } from 'xterm';
 import { FitAddon } from 'xterm-addon-fit';
+import { Dispatch } from '../actions';
 import { receiveData } from '../actions/terminal';
 import { terminalOutput } from '../epics/terminal';