[FEATURE][UX] Install Tool usability and visual enhancements

* Restyle toggle-groups with a more t3skin-like visual.
* Restyle "All configuration":
  * no more "green success boxes" per item, but instead
    alternating background.
  * no more horizontal rulers in between.
  * Clicking in the explanatory text in some item will focus the
    corresponding input-field.
* Submit buttons in "All Configuration" and "Configuration
  Presets" are now always visible due to them sticking to the
  bottom of the scrollable area.
* Use CSS3 gradients in buttons instead of background images.

Related: #52087
Resolves: #38701
Releases: 6.2
Change-Id: I1abfb42273bcb9f60c3feef9fcfbb411585c63bf
Reviewed-on: https://review.typo3.org/24187
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring