Backport popover code from Kiwix PWA

[Jaifroid]

Fixes #719. This backports the new feature from the PWA.

So far, I have ported the mouseover and the focus events, together with the popover code. Focus event provides keyboard support (tabbing into a link).

To do:

• [x] Add touch support (long press)
• [x] Add pointerdown fallback for browsers that don't support touchstart (e.g. Safari on macOS, and Edge legacy)
• [x] Fix hover over popover in Safari for macOS
• [x] Disable the contextmenu event if popovers are enabled
• [x] Enable for Safe mode (without image support, as in the PWA)
• [x] Add the option to disable popover preview in the UI, with translations
• [x] Fix support for dark modes
• [x] Move as many inline styles as possible to stylesheet
• [x] Suppress popover loading if user has clicked the link
• [x] Add e2e test for presence of popover on tab into link (Ray Charles ZIM)
• [x] Make the breakout icon open a new window
• [x] Fix clicking on a link inside the popover
• [x] ~~Add popover support to new windows~~ Open a new issue to add popover support to new tabs/windows: #1253
• [x] Clean up code hangovers from the PWA that are not needed here
• [ ] Final testing