fix: block signals modifying the store upon startup

[Doublonmousse]

Fixes #1258

Now for now I'm blocking/unblocking signals upon startup.

• I'm not 100% sure if the unsaved indicator is coming from the add_initial_tab and widget flags from this call
• Not all signal handler are blocked for now

These one can also change the store and are not treated yet

• doc_background_pattern_color_button
• doc_background_color_button
• set_document_layout calling to -> doc_document_layout_row

It also seems to me like we can either change the logic to set the initial values for the document section before the activation or setup of the handlers in setup_doc or change how the SignalHandlerId variables are held to only use them once (we probably don't need to reuse them later).

Seems like the unsaved indicator on startup is a little more common after the refactor on document settings as well