Accessibility: Fix incompatibility with MinGW

[mangobrain]

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WINE (and by extension MinGW) started adding new UI automation property IDs to uiautomationclient.h as macros, which conflicts with the named constants in JUCE's juce_win32_ComInterfaces.h. As there were already some older property IDs in JUCE defined as macros in juce_win32_AccessibilityElement.h, migrate all named constants to macros, for consistency & compatibility.

Fixes #1028