Prepare to add predictive filters

Predictive filters detect which value can be selected next for
an author to match.
Extract function getFilterExpression() from combineActiveFilters()
Extract function getMultiplier() from combineActiveFilters()
Move the definition of the two new functions

I moved the functions before getSelectorFunction() where I intend
to use them, replacing parameters with local variables.
Clarify: selector expression is now only used internally

I removed the comment about the selector expression in the
description of combineActiveFilters().
Replace parameters with local variables in getSelectorFunction()
Directly return a regular expression in getFilterExpression()
Start implementation of utility functions for predictive filtering
Move constants from combine-active-filters to filter-constants

The goal is to share these constants with a new module,
prepare-predictive-filters.
Move predictive filtering to new module prepare-predictive-filters
Rename combinePredicteFilter() to preparePredictiveFilters()
Complete description of "predictive-filter-function" property
Remove extra character from comment
Rename getSelectorFunction() to getActiveSelectorFunction()
Rename getFilterExpression() to getActiveFilterExpression()
Rename local variable filterExpression to activeFilterExpression
Rename getMultiplier() to getActiveMultiplier()
Rename local variable multiplierFilter to activeMultiplierFilter
Rename local variable multiplier to activeMultiplier
Share functions of combine-active-filters for reuse in predictive filtering