This mirrors the behaviour on macOS.

This commit removes the various compiler-specific JUCE_DEPRECATED macros and replaces them with C++14's deprecated attribute. It also removes the JUCE_CATCH_DEPRECATED_CODE_MISUSE flag as we can rely on the override specifier catching usage of these old virtual methods, and tidies up the DOXYGEN preprocessor checks as they were inconsistent across the codebase.

This fixes BadWindow errors from Expose events that may be processed after the window is destroyed.

On Linux, repaints are only issued if the number of received SHM
completion events matches the number of events initiated with
`xShmPutImage`.

Previously, destroying a GL context was calling `XSync (display, True)`
which cleared the event queue. Sometimes, this would cause an 'SHM
Completion Event' to be removed from the queue. This meant that the
number of received events would never match the number of initiated
events, and repainting would stop indefinitely.

By calling `XSync (display, False)` instead, we ensure that pending
events are kept in the queue, so any pending SHM completion event will
be found as usual during the next repaint timer callback.