Improve high DPI support in wxAui

Window initial/minimum/maximum sizes are now treated as logical pixels.

Furthermore, many margins and paddings are now converted using
wxWindow::FromDIP() to allow their growth in accord with screen DPI.
This places buttons on toolbars more apart on high DPI screens providing
space for easier touch operations.

Closes https://github.com/wxWidgets/wxWidgets/pull/933

src/aui/tabmdi.cpp

@@ -728,7 +728,7 @@ bool wxAuiMDIClientWindow::CreateClient(wxAuiMDIParentFrame* parent, long style)
     if (!wxAuiNotebook::Create(parent,
                                wxID_ANY,
                                wxPoint(0,0),
-                               wxSize(100, 100),
+                               parent->FromDIP(wxSize(100, 100)),
                                wxAUI_NB_DEFAULT_STYLE | wxNO_BORDER))
     {
         return false;