WebExtension placeholders tips

**Is your feature request related to a problem? Please describe.**
Currently

According to [WebExtensions of WebExtension](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Internationalization#Retrieving_message_strings_from_JavaScript), translations can have explanations of variables, it's called placeholders.

Example:
https://github.com/mdn/webextensions-examples/blob/master/notify-link-clicks-i18n/_locales/fr_FR/messages.json#L20-L25

Currently, this tips, are not shown into the Weblate UI.

**Describe the solution you'd like**

Obviously, it can be useful to display this information (in the good locale) to the translators.

**Additional context**
Also, I don't know if it's a bug but, the "Source string comment" contains the comment in the source language, and in the translated language:


I think I should be listed into two fields (and the fields from the translated locale should be editable).