fix: readonly-buttons-availability

[SystemChanger]

1. Fixed INDENT and OUTDENT button availability on toggling read-only mode + onClick/focus/etc.
   

2. Fixed undo and redo buttons availability on exiting read-only mode
   

[JiHong88]

@SystemChanger The branch is complicated, so I can't accept the PR.
This command is not executed..
git pull git@github.com:SystemChanger/SunEditor.git fix-readonly-buttons-availability

[SystemChanger]

@JiHong88 What do you mean by complicated? Could I see full log if provided?
There is a conflicting dist/suneditor.min.js file, which should be omitted on merge.
Also, you can fully allow changes for this file from the PR, because it is nothing more than a built js. I provide it in PRs so that you can quickly check the new changes without having to build by yourself.
It can be done through "Resolve conflicts" button, isn't it?

Will it be ok to use theirs/ours merge strategy?
git pull git@github.com:SystemChanger/SunEditor.git fix-readonly-buttons-availability -X theirs
or
git pull git@github.com:SystemChanger/SunEditor.git fix-readonly-buttons-availability -X ours

Looks like this behavior may be automated for repo by creating .gitattribues with merge strategy for dist folder:
How to Ignore Merge Conflicts for Specific Files in a Git Repository
Don't know whether it will affect the github conflict detection and GUI.

Also, I think there could be a way to automatically build on pull-request approval.