Integration unsuccessful when desktop file is symlinked

[Theodore Diamantidis]

The AppimageToolsRepository.integrate method expects the desktop file to be a regular file when moving it into $HOME/.local/share/applications/.

However, some images (e.g. KeePassXC) use a symlinked desktop file that points to usr/share/applications/ (allowed in the spec).

This causes the following when the user clicks on "Integrate":

• A symlink is copied to $HOME/.local/share/applications which however is dangling since it points to a usr/share/applications path that does not exist in that directory.
• The app is therefore never actually integrated.
• If the user clicks on "Disintegrate", the action fails with an error of:

  [ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: RangeError (length): Invalid value: Only valid value is 0: 1
  #0      AppimageToolsRepository.remove (package:appimagepool/src/features/home/data/appimage_tools_repository.dart:163)
  <asynchronous suspension>
  #1      AppimageToolsRepository.integrateOrRemove (package:appimagepool/src/features/home/data/appimage_tools_repository.dart:195)
  <asynchronous suspension>
  #2      InstalledViewController.integrateOrRemove (package:appimagepool/src/features/home/presentation/installed/installed_view_controller.dart:26)
  <asynchronous suspension>
  #3      InstalledViewState.integrateOrRemove (package:appimagepool/src/features/home/presentation/installed/installed_view_state.dart:40)
  <asynchronous suspension>
  

  that is caused by this line that expects the desktop file to have proper contents.