Allow platform to decide default behavior for show() based on window flags

The ShowIsMaximized and ShowIsFullscreen style hints were not granular
enough to build a default behavior from that would be correct for all
platforms. The recent Android patch that excluded dialogs from being
shown maximized (Ia249e93dbbea1) has now been moved into a platform
override in the Android integration plugin, leaving other platforms
to the default behavior of using the style-hints. We still special
case popup-windows though, as that behavior has been there for a
while.

Task-number: QTBUG-34969
Change-Id: Id36346d71bfc46171383ffe334592ca0b94e456f
Reviewed-by: BogDan Vatra <bogdan@kde.org>
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>
Workaround source compatibility issue introduced by xcb 1.9.3

Previous version of the struct:

typedef struct {
    uint8_t  response_type;  /**< Type of the response */
    uint8_t  pad0;           /**< Padding */
    uint16_t sequence;       /**< Sequence number */
    uint32_t length;
    uint16_t event_type;
    uint16_t pad1;
    uint32_t pad[5];         /**< Padding */
    uint32_t full_sequence;  /**< full sequence */
} xcb_ge_event_t;

New version of it:

typedef struct xcb_ge_event_t {
    uint8_t  response_type; /**<  */
    uint8_t  extension; /**<  */
    uint16_t sequence; /**<  */
    uint32_t length; /**<  */
    uint16_t event_type; /**<  */
    uint8_t  pad0[22]; /**<  */
    uint32_t full_sequence; /**<  */
} xcb_ge_event_t;

Changes are:
- "pad0" became "extension"
- "pad1" and "pad" became "pad0"

More details in https://bugs.freedesktop.org/show_bug.cgi?id=71502

Task-number: QTBUG-34748
Change-Id: Ibd801c11510f75fa82d5c14346b95236142487ac
Reviewed-by: Uli Schlachter <psychon@znc.in>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>