A lot of theme negotiators are initialized on every request

Problem/Motivation

Already standard install initializes the following theme nego.:

• 'theme.negotiator.system.db_update'
• 'theme.negotiator.system.batch'
• 'theme.negotiator.default'
• 'theme.negotiator.block.admin_demo'
• 'theme.negotiator.ajax_base_page'
• 'theme.negotiator.admin_theme'

even most of the time just 'theme.negotiator.default' is needed.

Proposed resolution

Consider to store which theme negotiatiors apply to which route.

Remaining tasks

User interface changes

API changes