Add Support to additional folders by changing reworking "Choose another folder" to "Add folder"

[sorbaugh]

Enhancement to #141

Reference: #141 (comment)

[artonge]

Quoting from the PR:

It is currently not possible to make a search on multiple folders with WebDAV. This limitation should be removed by nextcloud/server#42915.

[artonge]

• Server PR: Support multiple scopes in DAV search server#44858
• Photos PR: feat: Allow multiple source folders #2426