EL: improve resourcebundles handling

[NicolaIsotta]

This is a follow-up to #8603
The main aim is enabling CTRL + Hover and CTRL + click on ResourceBundle items in JSF pages:


The features were already integrated in ELHyperlinkProvider but it seems GsfHyperlinkProvider has the precedence so I had to implement them in ELDeclarationFinder (which is called by GsfHyperlinkProvider).
Aside from that, I tried to improve the resource bundled loading here and there, using the built-in cache whenever possible.

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