Avoid map/set double lookups

Fix some common anti-patterns with these data structures.

- You can dereference the iterator returned by `find` to access the
  underlying value directly, without an extra `operator[]`/`at`.
- Rather than checking for an element before insertion/deletion, you can
  just do the operation and if needed check the return value to
  determine if the insertion/deletion succeeded.

ClickBlurLabel: Create a label which is blurred until it is clicked

This is particularly useful for people who stream their desktop and don't want to accidentally leak their IP or room.

Remove window hints clearing the flag Qt::WindowContextHelpButtonHint,
which is already off by default in Qt 6.

In Qt 5 this flag was set by default for QDialogs, and on Windows put a
? button in the corner of the title bar allowing users to activate Qt's
QWhatsThis help system for a given widget. Since we don't set that text
the ? button was useless and so we hid it manually.

This reverts the revert commit bc67fc97c39628c76a4dbca411b0e8a9bfaf726a,
except for the changes in BaseConfigLoader.cpp, which caused the bug
that made us revert 72cf2bdb87f09deff22e1085de3290126aa4ad05. PR 12917
contains an improved change to BaseConfigLoader.cpp, which can be merged
(or rejected) independently.

A few changes have also been made based on review comments.