Issue 6880 - Made DevTools panel translatable

This MR for #77 (closed) contains the following changes:

• Created locale/en_US/devtools-panel.json and moved hardcoded strings in there.
  • After consulting with Tamara, added description for "devtools-header" string to provide necessary context for translators.
• Changed implementation to work with translatable strings.
• Introduced <slot#> element to translatable strings to declaratively place DOM elements in the resulting strings. See also <slot>. Please let me know if you think that this does not belong in the i18n module since it could be considered specific to the DevTools panel and because we may not want to encourage using it.
• Fixed/finished mock implementation.

Note that I tried not to refactor any code to keep the scope of this change in bounds but there's obviously quite a big potential for improvements regarding both code and design.