Update nl.po and some text suggestions

[peter-scholtens]

This is an update of the Dutch translation. Some suggestions for clarification of the original msgid texts:

• As for end-users the subtle distinction between removal of an indent (removing possible multiple characters) or a character may be unclear, please clarify like "With the cursor in the indentation, pressing backspace unindents (removing posible multiple characters) instead of deleting single character"
• A hyperlink is present four times in the text. Please add a GUI hyperlink instead : https://docs.gtk.org/glib/method.DateTime.format.html.
• The string "_Join Lines" doesn't explain the sequence of how the lines are joined. A description like "Concatenate Lines" would fit better I think.

[frlan]

@peter-scholtens Could you add your suggestions as atomic issues/mr so we ca discuss them? We most likely will not change them till 2.0 and you might can give some more background