Paste and Go Key sγur AQmx

Adds a changeable keyboard shortcut that pastes the clipboard into the URL bar of the current tab or a new tab and loads it if it is a URL or otherwise searches for it. The search function is disabled by default and the search engine that is used can be changed in the settings. Also includes a toolbar button.

Notes:
- If the search URL is left blank then searches will be done using the browser's default search engine.
- The file size is mainly due to the icon images; the actual code is about 15 kB.
- The shortcut key cannot be changed in Firefox 57 - 59.

Translation credits:
- Chinese Simplified - Jiong Guo (JEANX58)
- Italian - Elena Giannoni (ellieG)
- Portuguese - Rodrigo Leye (rleye)
- Swedish - Ben Skoglund (Mjuke)