Configurable scroll speed & Remembering last directory

* For some reason, scrolling of `Response -> Body` and `Request -> Body -> String body` (other scrollable areas seems to be unaffected) via mouse wheel and scrollbar arrows was slow. This effectively prevented using of mouse wheel to scroll at all. Fixed by adding option to configure scrolling speed explicitly.
* Load/save dialogs now remembers directory of a last selected file and starts with this directory as current.

Both of these are stored in persistent global options.