add hardcoded _schema_visible field to pipes and valves for enhanced …

[3nids]

…perfomance on QGIS

This creates a field _schema_visible which determines if an element is visible in schematic view. It is calculated by COALESCE(schema_force_visible, value_list.schema_visible). This allows direct use of this field in QGIS for rule-based labeling and fixes performance issues if calculation is done on client side.

• schema_visible field is dropped on network_element as it is not used (was introduced for valves at the origin)
• _schema_visible and schema_force_visible are added to pipe and valve tables by the qwat_sys.fn_enable_schemavisible function
• old function qwat_sys.fn_enable_schemaview is enhanced and renamed

[3nids]

this does not provide very noticeable performance gain. But this fixes a few names of triggers and brings a useful field in the tables (pipe/valves). @haubourg @ponceta any concern to merge this?