Fixes #738 #610 / Yi.Keymap.Emacs (moveE)

[dnsl48]

Checks isearching before trying to finish it.
Before the fix, moveE always tried to isearchFinishWithE, which made setSelectionMarkPointB $ regionStart p0 (Yi/Search.hs:390) and broke selection.

Although the problem has been localized and fixed here, there is the second place where the problem might show itself: https://github.com/yi-editor/yi/blob/master/src/library/Yi/Search.hs#L390
I'm not a vim user, that's why I suppose that this selection marking might be there for vim mode, but it makes the corruption of emacs's mode selection when isearch is being stopped.

[dnsl48]

Yi.Search:390 was added in 48acb13...59adc14.
Also, it had msgE "mark saved where search started" so that it wasn't used to any internal purposes of the editor. Looks like it was there for highlighting.
Furthermore, this function was made for the Emacs mode exclusively. I don't know if emacs behaviour was different in 2007, but now it's not the same, so I suppose that we can delete this marking painlessly.

[Fuuzetsu]

Seems to work, thanks. Squashed the commits into one because we don't need 3 fixup commits and also ammended the message slightly to adhere to usual git conventions