Various bugfixes

[juliusknorr]

For nextcloud/server#14892

• Debounce the input instead of asking for every character on the server
• Remove the % from the search query that is sent to the server
• Populate the collections search with some collections instead of waiting until the user typed something. (fixes Add hint about adding to existing collection #56)
• Error handling Show an error message if resource cannot be accessed when adding #57