[laurentiu021]

Apply on Environment Variables wrote the copy of a variable the tab had read when it opened, so a change made outside
the tab meanwhile was written over, and the new-variable picker offered "All".

What changed

• Apply reads every variable it is about to change again. Python, Node.js, Git and JDK installers add themselves
  to PATH, and the tab stays open for as long as SysManager runs. Moving one PATH row and pressing Apply wrote back the
  PATH the tab had read, and the installer's folder was gone. After the confirmation, Apply now reads every variable
  it is about to write or delete again through the new EnvironmentVariableService.TryReadValue, raw as Read reads
  it, and applies nothing, the safety copy included, when one changed, a new one appeared under the same name, or one
  could not be read again. The status line names them, "PATH (User) was changed outside SysManager after this tab read
  it, so nothing was applied. Press Refresh to see the change, then make yours again.", and the edit stays pending.
• A new variable is User or System. The "New variable scope" picker bound ScopeFilters, whose "All" made one
  User variable while the status line said "Added All variable 'X'". It binds NewScopes, User and System.
• EnvVariable.LabelOf names a scope the way ScopeLabel does, for the variables Apply has only a scope and a name
  for.
• CHANGELOG 1.130.58, README (Environment Variables), ARCHITECTURE (EnvironmentVariablesViewModel).

Tests

Over the redirected environment the other tests use, so no real variable is read or written:

• An installer adds a folder to PATH after the tab read it, a PATH row is moved, and Apply leaves the registry's PATH
  as the installer left it, writes no safety copy, says why and keeps the edit pending. With nothing changed outside,
  the same Apply writes.
• A new variable that appeared outside meanwhile, and a deleted one changed outside meanwhile, are left as they are.
• The refusal's wording for one, two, three and more variables; TryReadValue reads a value raw and returns null for
  none; NewScopes is User and System and the view's picker binds it.

Each test failed against the defect it pins: with no check, with new variables or deletions left out of it, with the
value read expanded, and with the picker bound to the filter's list.

Closes #2798.