Add an UI to select folder(s) existing in a specific configured Source

From the Server and Store creation or edit sheets:

• add a button next to Folder field,
• when clicked this button opens a pop-up with the list of folders found from configured source, with tree information (folders and sub-folders) (**)
• a checkbox is displayed in front of each folder, user can check multiple checkboxes
• all selected folders are used when loading emails from this source

how to manage, if it's in creation pop-up, the fact that it will be a second modal pop-up (feasible ?), and the fact that the XObject is still not saved (that should not be complex)
(**) how would it scale, if a very large number of folders exist ? (for mbox and for email accounts). JSTree extension + pagination + ajax ? (not so easy)
(***) what if user wants to configured a source and its folder, without access to it, at the moment he defines it ? (for example, no internet access, mbox file not yet saved, etc...). In this case, he should still have the possibility to specify a list of folders manually. Maybe so just by entering list of folders separated by ',' (comma) in the text field. So this text field should be synchronized with the folders pop-up, but if no folder is found the text input should not be emptied. And if a folder in the list does not (yet) exist in the source, it should not be removed. Saving the folders pop-up should then just add new folders, remove unchecked folders, and not replace blindly the list.