State saving and major rewrites

New:

• [UI] Save and restore calculator state across invocations
• [Converter] Save currency converter forex data after update
• [Calculus] Fix derivative accuracy bug which caused it to output zero every time

Translations:

• French (by Nathan Bonnemains)

Fixed:

• Code structure fixed and improved by Nathan Bonnemains
• [Calculus] Make Calculus button labels consistent with Scientific view

Improved:

• [Controls] Use Escape Key to close dialogues