Redo configuration window

The configuration window is re-done in a modern style. The changes include:

• Use Form Layout style
• Use standard style list delegates
• Use standard Kirigami Units for distances, sizes, and paddings
• Use an invokable function to remove paths rather than directly manipulating the model from inside the view
• Re-word some text for clarity and brevity
• Display warning for using filesystem crawling only when the option is selected
• Combine everything into a single file for simplicity
• Don't let the user remove the last music search path
• Re-order properties to conform to standard style
• Use QQC2 import name

These changes fix the following bugs: BUG: 425140 BUG: 418578 FIXED-IN: 21.04

Note that this bumps the frameworks dependency to 5.77 due to the use of the new trailing property in Kirigami.BasicListItem.

Here's how it looks now:

cc @teams/vdg