#15 Feature: thumbnails

This feature and the dependency (PIL) it needs are optional. ## This is on hold until https://notabug.org/CYBERDEViL/searx-qt/issues/16 is resolved (excluding this :')). **NOTE**: There are some HACKs used to display the thumbnails in a grid and still being able to set backgroud, borders, paddings, etc.. **Current TODO list**: - [x] Implement fetching images. - [x] Separate connection settings for image fetching. - [x] Display images in the search results. - [x] Option to cancel all active image downloading threads. - [x] Default loading/failed images. - [x] Re-try download thumbnail from context-menu. - [x] User feedback on why a thumbnail failed. - [ ] Remove weird hacks that should prevent race conditions and implement proper thread safety. - [ ] Use only 1 thread for image downloading/processing aka use pycurl and drop python-requests. - [ ] Testing and bug fixing. **Current known issues**: - [x] ~~Mixing normal search results with image search results may be a bit messy when there are alot of images. There might be large gaps between image blocks and text blocks.~~ Image results are now rendered after all other results, this fixed it. - [x] ~~The image grid view doesn't work on all systems, look into that.~~ (The issue was missing default theme)