Consolidate column filtering in M6

Previous description of issue from which the epic was extracted:

Expected results

• I don't have to expand items to see filtered items
• any hierarchy is not a performance issue (just the total number of items)

Actual results

• very deep hierarchy might suffer from perfomance issues MGNLUI-5806
• I can't see how many items is matching the filter MGNLUI-6088 
• expectation from tree filtering might differ PAGES-334 

Workaround

• Carefully read the docu https://documentation.magnolia-cms.com/display/DOCS62/JCR+data+source#JCRdatasource-JCRcomponentfiltering
• manually expand filtered items 

Development notes

After internal discussions, the most natural solution seems to be to show a flat structure in case of an active filter

• do not switch to list view
  • as columns might differ, size of columns might be manually adjusted
  • just display flat structure in tree grid
• follow up issues: expanded items status might be lost
  • separate context/UI state holding info on expanded items