[Dev] Refactor AddProgramSource dialog and bugfix

[VictoriousRaptor]

Changes

• Refactored logic of AddProgramSource dialog.

• For win32 and uwp ProgramSource (single programs disabled in results by users), Directory textbox is readonly and the Browse button is hidden. Users shouldn't change its location mannually since it's indexed by plugin, or it may lead to unexpected behavior.
  

• Fixed an issue that editing dir of the selected ProgramSource causes the selected item of ProgramSource listview can't be unselected.

• Removed unnecessary LocationConverter.

Tests

• Directory textbox is readonly and the Browse button is hidden for uwp and win32. Custom sources are unaffected.
• Can Enable/Disable Program Sources.
• Can Edit Location of Program Sources.
• Items can be unselected after editing directory.
• Location and Enabled column in listview are displayed correctly, after editing.

[jjw24]

I noticed:

1. The Program Source window does not show the path after selecting from the browse button.
2. Double clicking on the headers 'Name', 'Status' or 'Location' brings up the 'Please select a program source' message window.

[taooceros]

Sorry I think @jjw24 is right...Let's address those problem

[taooceros]

I fix the first one. The second one may require quite a bit more refactor to fit more mvvm scheme. There's a way to apply style to the item only, but which will overwrite the current style from modernwpf