Internet Content: Fix warnings about null strings during DB insertion
Internet Content: Fix manual update of RSS subscriptions

When triggered from MythFrontend, an RSS update would never complete, leaving
a busy dialog open forever.
MythNetvision: Move common stuff to base classes
MythNetvision: Fix some cppcheck warnings.
MythNetvision: Fix some issues found by Coverity
MythNetvision: Rename various methods
MythNetvision: Remove one member in NetTree
MythNetvision: Whitespace cleanup
MythNetvision: Fix thumbnail loading in the list view

Fixes #11800
MythNetvision: Simplify busy dialog handling
MythNetvision: Set progress dialog to NULL after closing it
MythNetvision: Split NetTree::SlotItemChanged into separate methods
MythNetvision: Set childcount and other text fields in the tree view
MythNetvision: Focus the search results after a search has been completed

Refs #12072
MythNetvision: Update thumbnail and text when switching search pages

Fixes #12072