Overhaul footer handling for simplicity & add option to disable showing update pages

Overhaul footer handling for simplicity

• This change separates the footers for different application modes, and moves much of the logic out of Main.
• Entirely refactoring with no visual change, but makes the footer logic more explicit and clear
• Strictly speaking, it also fixes the separator height not being accounted for which previously ate a row of pixels from the bottom of the page.

FooterUpdate: Add option to disable showing Plasma update pages

• Adds a switch to the update page footer.
• This is a switch because the action takes effect immediately, consistent with the usage in other parts of Plasma.