[3.x] #1957: Fix keepalive.

[haringsrob]

fixes #1957 on 3.x (to be ported)

[what-the-diff]

• Add destroyValue method to BrowserField, ConnectorField and LocaleField
• Call the new destroyValue methods in some components (Mediafield, Slideshow) when a field is destroyed or deleted
• Remove form fields from store if they are removed from DOM tree

[codecov]

Codecov Report

Base: 11.62% // Head: 11.62% // No change to project coverage 👍

Coverage data is based on head (634d2fa) compared to base (fecd20e).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff            @@
##                3.x    #2043   +/-   ##
=========================================
  Coverage     11.62%   11.62%           
  Complexity     3858     3858           
=========================================
  Files           302      302           
  Lines         14946    14946           
=========================================
  Hits           1738     1738           
  Misses        13208    13208           

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