More Application class documentation updates

* Completed documentation of the Application class:
    + Documented and grouped functions relating to
        - dialogs
        - application help
        - mnemonics
        - accelerators
        - VCL toolkit and UNO wrapper functions
        - IME Status functions
        - platform functionality
        - graphic filter functions
        - headless mode functions.
    + Documented event hook function.

* Documented the ApplicationEvent class

Change-Id: I79a77212aa64ef1d4d40bfed2b73af5dce357731
Reviewed-on: https://gerrit.libreoffice.org/7713
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>