Find/Replace Overlay: Support special common escape characters

[laeubi]

Sometimes one likes to "fixup" things, e.g. replace <tab> by <space> and under Linux GEdit it has support for \n and \t to be treated as a literal tab or newline.

In general this works with find replace, but one has to copy such character from somewhere as one can't type it into the box itself as the keybindings itself are already used (e.g. enter -> search, and tab -> switch to next input field).