Filter by installed

[BlazyNights]

I was annoyed that I wasn't able to filter down to what mods I had installed, this change adds that functionality.

Changes:
Pages/Mods.xaml:

• Installed header was changed to a checkbox, to try to stay consistent, I copied the values from the search header, although the margin was changed to allow the checkbox part to fit.
• I didn't spend long on the front end, I just wanted the UI to show that there was an option for this and to show the state of it.

Pages/Mods.xaml.cs:

• This uses ModsListView.Items.Filter, which is the same thing the text search uses, a lot of the code in this change is built around supporting the ability to use both filtering options at the same time.
• FilteredToInstalled and FilteredBySearch are both simple state tracking bools, they're true whenever you're using one of the filters.
• While SearchBar.Height == 0 was used in one spot, I tried to use the same logic again when the text search is called, but it doesn't seem to update fast enough and my breakpoints were never hit, which is the reason for FilteredBySearch now exists.