When a Modal is open keep focus on the Modal

[mtscout6]

You can currently "tab" off the Modal into elements under the Modal. This is both bad practice and required for Accessibility.

/cc @jportela

[citelao]

The code here is fairly robust, aside from relying on a constant selector string of tabbable elements (a[href], area[href], input:not([disabled]), ...).

Note, the selector string should be *[tabindex]:not([tabindex=-1]) at the end, instead of *[tabindex].

See here for an implementation in React (/shameless plug)

[jquense]

fixed in #810