Add SystemSymbol support to menu items and buttons

Menu items and buttons currently cannot use the curated SystemSymbol catalogue; only toolbar items can. Developers who want a Save, Add, Delete, Search, or Refresh glyph on a MenuItem or Button must supply their own image assets.

Add system-symbol support to menu items and buttons:

• MenuItem.Symbol As SystemSymbol — a semantic symbol shown beside the item text, alongside the existing Icon As Picture. Assigning a non-None symbol clears Icon, and assigning Icon clears Symbol (the same mutual exclusion toolbar items already use).
• Button.Symbol As SystemSymbol, Button.Icon As Picture, and Button.SymbolPosition As SymbolPosition (Leading, Trailing, Above, Below) — icons render at draw time as scalable vectors that inherit the foreground colour, so they stay crisp at any DPI and respond to themes, disabled state, and selection.

Symbols work in the Menu Bar Designer, the embedded menu editor in the Toolbar Designer, and the Window Designer's Control Inspector, and they persist in menu-bar and window layouts and in generated project code.