File explorer improvements

[LianaHus]

• [x] Add access to the context menu for plugins (#397)
• [x] Finish (https://github.com/ethereum/remix-project/issues/55)
• [x] cop/past/cut
• [x] Drag and Drop
• [x] copying and pasting between workspaces is not possible (https://github.com/ethereum/remix-project/issues/1317)
• [x] serach in files (https://github.com/ethereum/remix-project/issues/1630)
• [x] Add Publish to gist to context menu of each folder (https://github.com/ethereum/remix-project/issues/801)
• [x] Publish to GIST with a right click on multi selected files (https://github.com/ethereum/remix-project/issues/851)
• [x] Ask to include default set of files and folders for new workspace (https://github.com/ethereum/remix-project/issues/1102)
• [x] multiselection in FE (https://github.com/ethereum/remix-project/issues/1131)
• [x] Sync Editor and FE for current selected (https://github.com/ethereum/remix-project/issues/1405)