Implement setting file name when creating new files in the FOLDERS panel

[aesilevich]

When creating new files in the FOLDERS panel with the "New File..." -> "File" context menu, Nimble opens new tab with "untitled" name, and then opens file save dialog when trying to save new file.

Name of a new file should be set when creating new file in specified folder. Entering file name can be implemented in popup dialog or directly in file tree in the FOLDERS panel.