desktop: Use appropriate OUString functions on string constants

Change-Id: I8a7d6609466f450e0bb998457d8bf16525152091
extensions: Use appropriate OUString functions on string constants

Change-Id: Ief21a59fc58cdd6fb3398bbe5080ed5a53cb374e
ucb: Use appropriate OUString functions on string constants

Change-Id: I29c99157e9698e4af1d42a5c708cf4ec4753cf34
Extend loplugin:literalalternative to loplugin:stringconstant

Change-Id: Ie425af19019126b6a15ac03f52e32d186a46db35