Implement clipboard support on POSIX platforms with SDL2

Added Sys_FreeClipboardData(char*) so I don't have to copy the string
from SDL_GetClipboardText() into a Mem_Alloc() buffer, but can just
do the right thing per platform, which in case of POSIX/SDL2 is
SDL_free().
SDL1.2 doesn't have clipboard support, otherwise I'd have removed all
platform-specific implementations and used SDL_Get/SetClipboardText()
everywhere (IIRC AROS only supports SDL1.2?)

neo/framework/EditField.cpp

@@ -504,7 +504,7 @@ void idEditField::Paste( void ) {
 		CharEvent( cbd[i] );
 	}
 
-	Mem_Free( cbd );
+	Sys_FreeClipboardData( cbd );
 }
 
 /*

neo/sys/aros/aros_dos.cpp

@@ -238,6 +238,10 @@ char *Sys_GetClipboardData(void) {
     return NULL;
 }
 
+void Sys_FreeClipboardData( char* data ) {
+	// as Sys_GetClipboardData() returns a static buffer, there's nothing to free
+}
+
 void Sys_SetClipboardData( const char *string ) {
     struct	IFFHandle	*IFFHandle;
     BOOL	written = FALSE;

neo/sys/posix/posix_main.cpp

@@ -49,6 +49,8 @@ If you have questions concerning this license or the applicable additional terms
 
 #include "sys/posix/posix_public.h"
 
+#include <SDL.h> // clipboard
+
 #define					COMMAND_HISTORY 64
 
 static int				input_hide = 0;
@@ -331,12 +333,28 @@ ID_TIME_T Sys_FileTimeStamp(FILE * fp) {
 }
 
 char *Sys_GetClipboardData(void) {
+#if SDL_VERSION_ATLEAST(2, 0, 0)
+	return SDL_GetClipboardText();
+#else
 	Sys_Printf( "TODO: Sys_GetClipboardData\n" );
 	return NULL;
+#endif
+}
+
+void Sys_FreeClipboardData( char* data ) {
+#if SDL_VERSION_ATLEAST(2, 0, 0)
+	SDL_free( data );
+#else
+	assert( 0 && "why is this called, Sys_GetClipboardData() isn't implemented for SDL1.2" );
+#endif
 }
 
 void Sys_SetClipboardData( const char *string ) {
+#if SDL_VERSION_ATLEAST(2, 0, 0)
+	SDL_SetClipboardText( string );
+#else
 	Sys_Printf( "TODO: Sys_SetClipboardData\n" );
+#endif
 }
 
 /*

neo/sys/sys_public.h

@@ -101,6 +101,7 @@ void			Sys_Quit( void );
 
 // note that this isn't journaled...
 char *			Sys_GetClipboardData( void );
+void			Sys_FreeClipboardData( char* data );
 void			Sys_SetClipboardData( const char *string );
 
 // will go to the various text consoles

neo/sys/win32/win_main.cpp

@@ -570,6 +570,10 @@ char *Sys_GetClipboardData( void ) {
 	return data;
 }
 
+void Sys_FreeClipboardData( char* data ) {
+	Mem_Free( data );
+}
+
 /*
 ================
 Sys_SetClipboardData