[find/replace] Add accelerators

Closes #19
[find/replace] Fix TAB navegation on the 'replace' field
[find/replace] Add 'interpret escape sequences' check and other minor things

- The new option transforms '\\n' to '\n' etc.
- If 'find next' or 'find previous' is used without having introduced a
  find string in one of the find dialogs but text is selected, search for
  that text (instead of showing the quick-find bar).  Same for 'replace
  next' if there's already a replace string.
- 'replace all' in a single undo action.
[find/replace] Change behaviour when activating the dialog with text selected

Previously the find or replace field (depending on hotkey) was updated
with the selected text.  Now if the text spans multiple lines (after
stripping whitespace) 'only in selection' is checked instead.

Closes #20