Tooltip: Rewrite the positioning logic of popupMixins.js

[yamgent]

Tell us about your environment

• MarkBind Version: The version after 1.10.0

What did you do? Please include the actual source code causing the issue.

As specified in issue #338, there was a tooltip positioning issue.

However, it was difficult to disentangle the logic inside popupMixins.js because it is spaghetti code, so it was hard to explain the logic behind the calculation of the tooltip position.

A temporary fix was done in PR MarkBind/vue-strap#87. It uses the mouse cursor position to fix the issue. However, we lose the original aesthetic of the tooltip provided by Bootstrap (the arrows in tooltip is gone + the placement attribute no longer matters now).

A more permanent solution would be to either rewrite popupMixins.js, or further experiment with Bootstrap-Vue's tooltip to see how we can modify it to suit MarkBind's needs.

[ang-zeyu]

Closed by #1033