preparation for mapping editor
Early mapping editor

Implements these features:
mapping lines showing the mapping
manipulate the mapping lines by drag and drop
filter values ... filtered rows should be mapped to NaN  (partially)
show mapping of individual data items
Fixed editor not returning new scale correctly

It did work before the previous commit, but I had messed it up and not
checked it before committing
Data item mapping lines, documentation, variables

Fixed issue with data item mapping lines and improved documentation and
variable naming
Revert "Data item mapping lines, documentation, variables"

This reverts commit 3c1a00b0ab9a84a77fe64a2901a12b3ac7638cea.
Revert "Revert "Data item mapping lines, documentation, variables""

This reverts commit 04901bfc45a0c0af98ebb03cc35f09754150f95c.
refactor and feature add
data overlay (feature add)
some stuff -- making it a bit fancy
small refactorings:
 * extract css to file
 * remove unused js + css files
remove event-manager.js
fix some warnings (missing ;)
avoid not needed parameter
simplify and format code
extract common popup setup to a function
extract header and body html ids to config
fix #18 text overflow in text columns
bugfixing
format source code according to WebStorm
bugfixing