Run mode over the whole line in long lines when in background-processing mode

Issue #1908
Fix completely botched IE11+ check
Export lineNo utility

Issue #1969
Make lineNo a method after all

Issue #1969
Add (undocumented) wholeLineUpdateBefore option

Issue #1972
Issue #1967
[placeholder addon] Don't hide placeholder text on focus

Most modern browsers now keep displaying the placeholder even if the
user has focus on the input and only hide it when the input has content.

This change is to make this addon consistent with this, so users won't
tell the difference with native inputs' placeholder behaviour.
Mark release 3.20
Bump version number post-3.20
[the-matrix theme] Fix broken CSS selector, tune down selection contrast.
Fix bug in patching lines with widgets

The gutter element would end up before any 'above' widgets, causing
the widgets to hide it.

Closes #1971
[changemode demo] Really rate-limit looksLikeScheme() calls
Add a wrapper class to gutters to offer more control using CSS.
[show-hint addon] Emit "pick" event when a completion is picked
[pegjs mode] Quotes can appear in character classes

when they do so, they don't indicate the start of a string.
Make textarea contextmenu shim have background: transparent

To prevent it from being visible on dark backgrounds.
[commend addon] Prevent bogus uncomment when markers not at line start

Closes #1975
Prevent infinite recursion in lineIsHiddenInner

Closes #1984
[website] Make things more or less run on IE8

HTML5 tag parsing hack, and invert css media query.
[php mode] Make double-quoted string hack also work for single-quoted

Closes #1992
[dialog addon] Add value option