easier methods of adding/merging more datasets with the active session dataset

[x-CK-x]

this includes the gallery tab and custom dataset tab; and options such as:

• add single post data from an image booru
• add/upload images from local machine
• download & merge in more data from the downloader tab into the active session rather than starting a new one

also this could be where the autotag/autocaption images feature could be useful from within the gallery than on a separate page; and this could pave the road for a more complete automated system of tagging/captioning images and pruning them.