modest-ui-actions.c: some alignment in the code

modest-ui-actions.c: some alignment in the code
modest-ui-dimming-rules.c: some cleanup of unnecessary codes
modest-msg-view-window.c: partially fixes bug 60714, and have further
modifications. The concept is using signals of tree_model for any
change. If there is a change, dimming rules should be applied on
msg-view-window.

Not: There remains some bugs, which I do not know if registered on
bugzilla or not, but are near the same problems like 60714.
The tree_view (header_view) in main window might have new tree_model
due to some user actions and so 60714 bug become current again.

pmo-trunk-r2959