Added mwv: URIs to allow opening the app directly to a specific page

[YourOrdinaryCat]

Title. The specific URIs for now are (each starting with mwv:):

• system: Launches SystemPage
• theme: Launches ThemePage
• links: Launches LinksPage
• Anything else: Launches AboutPage

There's also a few changes I made to app startup, notably changing the default min size. You can't use min dimensions larger than 500, so I constrained the initial height a little :) other than that I just added an InitializeView function to the App to make things easier. Oh and NavigationCache for MainPage.