Add option to display tooltip on link hover (#8394)

* Add option to display tooltip on link hover

This makes it possible for platforms like Electron apps, which lack
a built-in URL preview in the status bar, to enable tooltip previews
of links.

Relates to: vector-im/element-web#6532
Signed-off-by: Johannes Marbach <johannesm@element.io>

* Gracefully handle missing platform

* Use public access modifier

Co-authored-by: Travis Ralston <travpc@gmail.com>

* Use exact inequality

Co-authored-by: Travis Ralston <travpc@gmail.com>

* Document getAbsoluteUrl

* Appease the linter

* Clarify performance impact in comment

Co-authored-by: Travis Ralston <travpc@gmail.com>

* Use URL instead of anchor element hack

* Wrap anchor in tooltip target and only allow focus on anchor

* Use optional chaining

Co-authored-by: Michael Telatynski <7t3chguy@gmail.com>

* Use double quotes for consistency

* Accumulate and unmount tooltips and extract tooltipify.tsx

* Fix indentation

* Blur tooltip target on click

* Remove space

* Mention platform flag in comment

* Add (simplistic) tests

* Fix lint errors

* Fix lint errors ... for real

* Replace snapshot tests with structural assertions

* Add missing semicolon

* Add tooltips in link previews

* Fix copyright

Co-authored-by: Travis Ralston <travpc@gmail.com>
Co-authored-by: Michael Telatynski <7t3chguy@gmail.com>

src/BasePlatform.ts

@@ -231,6 +231,14 @@ export default abstract class BasePlatform {
         }
     }
 
+    /**
+     * Returns true if the platform requires URL previews in tooltips, otherwise false.
+     * @returns {boolean} whether the platform requires URL previews in tooltips
+     */
+    public needsUrlTooltips(): boolean {
+        return false;
+    }
+
     /**
      * Returns a promise that resolves to a string representing the current version of the application.
      */