refactor signatureguiutils.cpp to avoid unnessary page loops

documentSignatureMessageWidgetText and getSignatureFormFields have similar loops to extract PDF form fields from pages. I made both loops more similar and added early exit loop conditions to improve performance in documents with many pages

I have commited last time to KDE in 2012 or so and when I used the web editor of KDE Invent and hit commit, it really committed directly to master and I fear I broke it (pipline indicates that compile fails). Sorry! I expected it creates a personal branch and invites me to make a pull request.

Afer I reverted the original commit d2ae4302, this MR seeks to reintroduce the code changes.