Contributed by Dror Ben-Gai

Courtesy of Łukasz Frankowski

Closes #138

This would be visible in the activeline demo, where the colored line would get
shorter and shorter for each display update.

Also fixes a bug with computing the max width and height for placing the widget

Can be one of 'below' (default), 'over', or 'fit', and controls the way
the widget is positioned in relation to the given character position.
'fit' means CodeMirror does its best to keep the widget inside the current
dimensions of the editor.

To help external code easily determine which mode a getTokenAt token is in

Just show the default menu

This was, it doesn't have to be called during init, preventing the editor
from stealing focus when created.

Instead of hard-coding all kinds of language-specific behaviours in the
mode itself, specializations can now pass character hooks to implement
their own highlighting.

Using normal replaceRange causes an infinite loop when replacing something
with a string that contains the original string.

Even when the match failed because we fell off the end of our search range

Clicking can cause the textarea to become briefly unfocused, losing
the shiftSelectiing flag in that case is a bug.

Closes #121