Clip scroll positions given to scrollTo to scrollable space

Closes #1934
Add more extending unicode ranges
Document execCommand

Issue #1935
[dialog addon] Add cm.openNotification: timed & unobtrusive dialog
[dialog addon] Slight modifications to openNotification

Remove callback argument, save currently open dialog in per-editor state.
[vim mode] use openNotification instead of openConfirm for errors
[markdown mode] `\n* ` should not toggle em state.

Closes #1920.
[real-world uses] Add MVC Playground
[clike demo] Fix duplicate variable names for C and C++ editors

The C editor and the C++ editor have the same variable name, so
the latter shadows the former. Fixed by renaming both.
[css mode] Fix more indentation issues.

Closes #1925.
Only call onFocus when the editor is really focused
[go mode] Add ')' as electric character.

As a result, the common pattern

import (
	"foo"
	"bar"
	)

will re-indent automatically to the gofmt-blessed

import (
	"foo"
	"bar"
)
[htmlmixed mode] Fix bug in intra-token-closetag detection

Issue #1943
[show-hint addon] Fix event delegation for UL/LI

It's possible to provide custom markup for hint elements that are LI
tags and that breaks event delegation logic that expects UL to have
text-only LI nodes. As a result if you have elements inside LI and
click/dblclick on those, nothing happens and selected hint is not
applied. The fix improves event delegation in the way to find nearest
parent element to event target that is a hint element at the same time.
[show-hint addon] Style fixes

Issue #1947
[show-hint addon] Allow a specific option to be selected by default
[real-world uses] Update link to Handcraft
[markdown] Emit 'header1'..'header6' styles in addition to 'header'.
[ambiance theme] Text selection uses default background color

`cm-s-ambiance` and `CodeMirror-focused` are classes to the same
element, so search for `.cm-s-ambiance .CodeMirror-focused` will fail.
[docs] Remove underlines for headers