Improved tabbed mode

[User request]

While it's possible to show multiple documents in tabs, opening a new file always creates a new window and they need to be manually combined.

If the tab bar is visible, it would make sense that opening a new document opens a new tab, possibly with a separate command to open a document in a new window or tab as required.

Notes:

• ⌘T currently opens the font picker. It should open a new tab.
• It should be possible to open an empty window or tab with no file selected. The empty document shows the recent files list and a button to open the file picker.
• This empty window should be the default when the app is opened.