Newfeat 17 hide show blocks

[SaraBianchi]

New feature already explained here #5157 (I'm opening a PLIP about this).

Some improvements made compared to the old version:

• Accessibility of opacity text, now the color contrast passes

• The "show/hide" button has been positioned at the top center of the block, when the block is hidden, the focus on the block has a dotted border to better identify that it has been hidden
  

• checked layout also on mobile

New feature added

• an on/off switch in the left side toolbar to hide or show blocks preview from the page, hidden ones are collapsed, if there are no hidden blocks the switch is disabled.
  https://github.com/user-attachments/assets/8027ed8e-e6d3-47be-80cc-74208feb0010

Only sore point

• When the switch is activated, when a block is hidden and the next one is deleted by the keyboard, the focus is positioned on the collapsed one, if it is a slate text block it is possible to edit/delete it even while the content is hidden, it is necessary to expand it or exit with another key combination if you are not using the mouse.
  A subsequent improvement could be to tell the Slate text field to bypass hidden blocks if you use the keyboard only when they are collapsed.