Merge File and CSV File Import widgets

[irgolic]

Is your feature request related to a problem? Please describe.
In an effort to reduce the number of widgets, I believe the CSV File Import widget's functionality should be moved to the File widget. It is intuitive to use the File widget to load a CSV file.

Describe the solution you'd like
Should a CSV file be chosen in the File widget, the CSV import window opens in the same way it does currently in the CSV File Import widget.

[irgolic]

The biggest issue here is that you can't pick a target attribute in CSV File Import, the user needs to know to use Select Columns.

[janezd]

When merging, keep File's RecentPaths (see #4784).

[irgolic]

We'd like to keep CSV File Import, over File.
Missing functionality:

• Built-in domain editor
• URLs
• .xlsx (and other)