Consider consolidating UI helpers into a Microsoft.Terminal.UI namespace/dll

[DHowett]

work branch https://github.com/microsoft/terminal/tree/dev/duhowett/m.t.ui

We have some UI helpers flung all about the Terminal code, including in Terminal.Settings.Model, that could be consolidated.

• TerminalApp
  • EmptyStringVisibilityConverter
• TerminalSettingsEditor
  • Converters
    • Including a duplicate EmptyStringVisibilityConverter equivalent; deduplicate these!
• TerminalSettingsModel
  • IconPathConverter

I originally authored it to contain a resource string markup extension as detailed in an APS blog post. That'll help us get rid of all the places where we load resource strings in code because they couldn't be bound in XAML.

[DHowett]

We could also park some shared UI components there, which we can finally use from TSE and TA without copying them.

[DHowett]

And perhaps IDirectKeyListener, if we keep M.T.UI top-clean.