wxWindow: Mother of all visible GUI objects
wxFrame: A wm window (not a simple wxWindow) resizable by the user, with a title bar
wxDialog: Similar to wxFrame, but can be modal.
wxPanel: A wxWindow where the controls (other wxWindows) are placed. Like a
Dialog placeable *not only* in a new wm window.

Sizers
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Any wxWindow can have a sizer: wxWindow::SetSizer().

Features:
- Border: pixels around each internal control
- Minimal Size: set by each internal control
- Alignment:
- Stretch factor: (specified as 'proportion' to some sizers)
  - 0 : always minimum space for each control.
  - >1 : proportional size related to the sum of all control stretch factors.