begin a helper class for filtering

This should help shortening code and making it more readable. The
code filtering DB results defines its possible filters using this
class. Later on, it passes the actual filter data to it and recei-
ves the ready-to-use MySQL condition as result.
Use new FilterHelper class for suspension filters

One last filter is still done using the old method as
FilterHelper is not yet ready to handle such complicated
conditions.
fix FilterHelper value conversion

Always escape strings for security, therefore pass
a connection ID. Also fix int and bool conversion.

Remove outdated comment.
FilterHelper: escape column names always
fix null-check handling: was inversed
Suspension filters: remove duplication

Filtering for user has been redefined with the new class, so
remove its literal inclusion in the query. Thus, the special
prefix is no longer needed.
Rewrite candidate list filters with new class

Again, one condition is still implemented in the old way
as FilterHelper can not yet handle table joins.
undo meaningless whitespace change
cleanup FilterHelper code by breaking it into several methods
simplify suspension listing by removing duplication (@36d7ad9fb6)
massive changes to FilterHelper to support more complex conditions

Pass the DB connection to the constructor. Split up into further
methods. For filters of type 'switch', allow a set of conditions
to be met. Validate comparison parameters. Add a new 'expr' type
and some structuring comments.

Use the modified FilterHelper class to re-implement the last pa-
rameter of the Suspension class. Adjust the filter code for can-
didate listing.
extend FilterHelper to handle table joins

Implement the last candidate filter with FilterHelper.
Adjust all calls as necessary.
minor code shortening for FilterHelper
Shorten FilterHelper with new getValue() method
support values of type "custom" for filters

A coerce callback must be specified.
support REGEXP operator for filters
Convert item list filters to FilterHelper class

WARNING: This changes the outdated behaviour that the 'user'
filter took user names instead of IDs.
convert stdlib items filtering to FilterHelper class
Merge branch 'rework-filters' into master