本修改合并自 fex-team/kityminder-editor@db45c1fae。

This provides previewing GFM within the note editor, without hovering on the
node's "Note" icon. Toggle preview by pressing the "Toggle preview" button
on the left of the "Close" button.
Previewer state will be kept during a session.

Study Notes:
- Use ng-class to dynamically change an element's appearance (for creating
switch button)
- Use ng-bind-html to show HTML with the principle of AngularJS
- Know about how ng-show, ng-init and ng-click works. By using them we
won't need to write extra codes for controllers.

References:
- Create a switch button with AngularJS: https://www.shuzhiduo.com/A/D854LW9w5E/
- Show HTML content properly with ng-bind-html: https://www.cnblogs.com/lyy-2016/p/7928728.html

The dist/ folder will change on each build/dev process.
You should FIXUP such commits which update the dist/ directory.

dist update: 2020/11/29

比较自ACTom提交版本8f89e6cfe0b195e。

主要内容：
- 加入德、法、西、意与繁体中文的支持
- 进一步多语言化一些部分，包括新建节点、优先级图标、进度图标、搜索框。