Add a option to search for all available pdf files in the drive and add them to PDFkeeper

[ltolstoy]

Is your feature request related to a problem? Please describe.
Current procedure for adding new pdf files requires either add them one at a time, or at least provide one folder with pdf files in them.

Describe the solution you'd like
I would like to have an option for pdfkeeper to search for all pdf files available on the drive, even in separate nested sub-folders (or accept as an input a pre-made txt file with the list of pdf files with full paths to them, if it's easier. This file can be created with a help of app like Everything, which can quickly search for all pdf files on the drive), and process or index them all into local (or external) db. Then for each file db would keep the location on the disk, together with all important metadata.

Describe alternatives you've considered
Few options for this feature: allow command line interface for reading a pdf file (this way it is possible to create a script that will feed pdf files one-by-one from the large list, to prdkeeper to index them, keeping original pdf file location unchanged), or
allow pdfkeeper to accept as input a large pre-made txt file with a list of pdf files with full path to each of them, to index into db, or
allow pdfkeeper itself to search for pdf files on the HDD, generate this list of files in its own format (or plain txt), and index all found pdf files into db.

Additional context
This feature would greatly simplify the indexing process, I think.