[FEATURE] Allow appending of element browser mounts

A new User TSconfig option is introduced that allows the alternative
element browser page tree mounts to be appended instead of beeing
replaced:

options.pageTree.altElementBrowserMountPointsAppend

Resolves: #50780
Releases: 6.3
Change-Id: I2a0b616b398131684a2c7c71a25b4b51ef6f0c50