GSoC-2022: Add a New File menu item and the code for the New File dialog

Add a New File menu item which opens the New File/Templates dialog depending on whether there are templates in the XDG Templates directory.

Blog post detailing my work for the GSoC project: https://ignapk.blogspot.com/2022/09/gsoc-2022-overview.html

TODO:

• implement switching between the New File and Templates dialogs
• fix renaming new files
• allow choosing other files apart from blank text
• show the specific file type only if there's an app that can handle it
• hide the New File menu item if there aren't any apps that can handle the file types
• AdwEntryRow? AdwClamp?

Replaced !894 (closed), follow up to !888 (merged) See #2205 and Teams/Design/whiteboards#68