Improve URL placeholder in first wizard page

[fmoc]

Fixes #9713.

[ownclouders]

Results for GUI-tests https://drone.owncloud.com/owncloud/client/11885/6/1

💥 The GUI tests failed.

GUI Logs: https://cache.owncloud.com/public/owncloud/client/11885/guiReportUpload/index.html

Server Logs: https://cache.owncloud.com/public/owncloud/client/11885/guiReportUpload/serverlog.log

Screenshots:

• Adding_account_with_self_signed_certificate_for_the_first_time.png
• Adding_account_with_wrong_credentials.png
• Adding_multiple_accounts.png
• Adding_normal_Account.png
• Both_original_and_copied_folders_can_be_synced.png
• Connect_account_with_manual_sync_folder_option.png
• File_with_long_name_(>228_characters)_is_blacklisted.png
• File_with_long_name_can_be_synced.png
• File_with_spaces_in_the_name_can_sync.png
• Filenames_that_are_rejected_by_the_server_are_reported.png
• Invalid_system_names_are_synced_in_linux.png
• Many_subfolders_can_be_synced.png
• Sync_all_is_selected_by_default.png
• Sync_only_one_folder_from_the_server.png
• Syncing_a_folder_to_the_server.png
• Syncing_file_of_1_GB_size.png
• Syncing_folders_each_having_500_files.png
• Verify_one_empty_folder_with_a_length_longer_than_the_allowed_limit_will_not_be_synced.png
• Verify_pre_existing_folders_in_local_(Desktop_client)_are_copied_over_to_the_server.png
• Verify_that_you_can_create_a_subfolder_with_long_name.png
• change_collaborator_permissions_of_a_file_&_folder.png
• remove_an_account_connection.png
• remove_the_only_account_connection.png
• share_a_file_with_many_users.png
• sharee_creates_a_file_and_a_folder_inside_a_shared_folder.png
• sharee_deletes_a_file_and_folder_shared_by_sharer.png
• sharee_edits_shared_files_and_again_try_to_edit_after_write_permission_is_revoked.png
• sharee_renames_the_shared_file_and_folder.png
• sharee_tries_to_create_a_file_and_a_folder_inside_a_shared_folder_without_write_permission.png
• sharee_tries_to_delete_shared_file_and_folder_without_permissions.png
• sharing_a_folder_by_public_link_with_"Uploader"_role_and_check_if_file_can_be_downloaded.png
• sharing_of_a_file_by_public_link_and_deleting_the_link.png
• sharing_of_a_file_by_public_link_with_password_and_changing_the_password.png
• simple_sharing_of_a_file_by_public_link_with_default_expiration_date.png
• simple_sharing_of_a_file_by_public_link_with_password_and_expiration_date.png
• simple_sharing_of_file_and_folder_by_public_link_with_expiration_date.png
• simple_sharing_of_file_and_folder_by_public_link_with_password.png
• simple_sharing_of_file_and_folder_by_public_link_without_password.png
• simple_sharing_of_folder_by_public_link_with_different_roles.png
• sort_folders_list_by_name_and_size.png
• unshare_a_reshared_file.png
• unshare_a_shared_file_and_folder.png
• various_types_of_files_can_be_synced_from_client_to_server.png
• various_types_of_files_can_be_synced_from_server_to_client.png

[fmoc]

Regarding the placeholder pointing to an actual domain, should we actually allow an empty value and fall back to the placeholder value?

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information

[TheOneRing]

Regarding the placeholder pointing to an actual domain, should we actually allow an empty value and fall back to the placeholder value?

Its only a hint, doing weird stuff if a user presses enter is ux from hell.
We could prefill the lineedit but also bad ux.