Make tab index on tabline consistent. 
**Is your feature request related to a problem? Please describe.**

e.g. We have the following tabs, and we are in tab 4 javacomplete.vim

```
<- 10 | 1 foo.vim | 2 Foo.java | 3 SpaceVim.vim | 4 javacomplete.vim | 5 util.vim | 13 ->
```

``\5`` jump to tab ``util.vim``, tabline becomes:

```
<- 13 | 1 javacomplete.vim | 2 util.vim | 3 tab1.txt | 4 long_tab2.txt | 5 tab3.txt | 10 ->
```

We notify the tab index is changed.

``\1`` return to tab ``javacomplete.vim``, tabline maybe
```
<- 10 | 1 foo.vim | 2 Foo.java | 3 SpaceVim.vim | 4 javacomplete.vim | 5 util.vim | 13 ->
```

The tab index change make it hard to jump between tabs.

e.g. The``PatchReview`` plugin opens one tab for each file (tens of files), and I need to frequence swtich between .h/.cpp.  The inconsist tab index make it hard to jump.

**Describe the solution you'd like**
Remember the left most shown tab index, and try keep it consistent, not jump consider previous tab index and current tab index.

BTW, if the ``gr`` PR(#3728) can be merged, it should helps a lot.
