implements overview widget for SPLs

including min/max/avg values for important metrics and some stats on
metrics (variable count, file instances,...)
clean up source of spl_overview widget
add metric descriptions in plain text

Each metric column header is annotated with a title attribute describing
its meaning.
add HOM/HET/HOHE dot-chart over all releases
fix logging of url paramters
pass spl specific GET parameters to widget

The 'file' and 'release' GET parameters are passed to widget instances,
wrapped in the list 'w$spl', as reactive values
add release-level widget for AND metric
implement info widget for single spl file instance
support dynamic max-values in gauge widgets

Updated justgage.js library to current version from
https://github.com/toorshia/justgage. Adapted output binding to support
passing of max-value with value updates.
add gauge widgets for file-instance metrics
add legend to HET widget on project level
adapt style of SPL summary widget