Editor: Implemented simple Find functionality

The missing find functionality turned out to be an itch that needed scratching for me. This was a very quick hack, and having never worked with Qt (or hardly any UI development) before it certainly needs to be reviewed.

In short, it adds a text input panel for find, that appears upon execution of a find command. The panel is hidden by pressing ESC with the cursor in the find input field or by clicking the "Done" button. Forward and backward searching (Ctrl-G,Ctrl-Shift-G) as well as "use selection for find" (Ctrl-E) are implemented.

It has only been tested on OS X, where it seems to work fine.

Replace functionality is not implemented, so it only addresses half of issue #106. Personally, I usually find it faster and more convenient to instead use a combination of Ctrl-E, Ctrl-C, Ctrl-E and Ctrl-V. Replace would be quite quick and straight forward to implement on top of this, however.