9LTS: Replacement for Ext.JS 3.4.1.1 code

[thomaszbz]

TYPO3 version: 8 LTS

As of MetaSEO 2.0.3, we use ExtJS 3.4.1.1 to render the backend modules.

As of current master, only the pagetree is left in the core: https://github.com/TYPO3/TYPO3.CMS/tree/master/typo3/sysext/backend/Resources/Public/JavaScript/extjs

We need a concept for TYPO3 8 LTS, now that ExtJS is supposed to be removed from the core. Basically, we have the same situation as earlier for 7.6 LTS (which luckily still got shipped together with ExtJS 3.4.1.1), as discussed via #114.

For instance, we have decided to port the Ext.JS code to vue.js in the backend, as suggested by @mediaessenz. This will be made available in metaseo 3.0.0, which supports TYPO3 CMS 8 LTS only.

Basically, that means that nothing changes for users of metaseo 2.x.y (which is for 6.2 and 7.6). So we should be able to avoid any sort of side effects for 6.2 and 7.6.

@mediaessenz is currently working hard to make this port available, which would in turn represent a major step towards 8 LTS compatibility. We cannot thank enough for such a large code contribution, which will literally help to keep the whole project alive.

[thomaszbz]

Seems as if 8LTS still ships ExtJS, thanks to the everlasting PageTree ;-) For instance, we'll stuck with ExtJS. That keeps the project somehow alive, too ;-)