Add filtering of numerical cols to collection

CollectionFilterWidget: Updated the tooltip, to reflect the changes.
CollectionQuery: Add parsing for SQL operators and insert right SQL
"where" searches.
Song: Add list of numerical columns
playlistfilterparser.cpp/FilterParser: move time and rating parsing
functions to new file:
searchparserutils.cpp: Contains common code used to parse search terms
in playlist and collection filters.

src/core/song.h

@@ -116,6 +116,8 @@ class Song {
   static const QString kBindSpec;
   static const QString kUpdateSpec;
 
+  static const QStringList kNumericalColumns;
+
   static const QStringList kFtsColumns;
   static const QString kFtsColumnSpec;
   static const QString kFtsBindSpec;