[TF2] Support for more than one activity in a sequence.

Having to specify the same animation over and over again for different activities (like one animation for running with a primary weapon and specifying the same exact sequence again but for a melee weapon) leads to file size bloat. Sometimes people like to include custom models in their maps but unfortunately the player has to specify multiple sequences for what is the same animation thus wasting space.

So instead of this for example:

$sequence "PLAYER_run_item1" {
	"anims/pm0740_00_fi21_run01.smd" 
	activity "ACT_MP_RUN_ITEM1" 1  	
	fps 60
	loop
}

$sequence "PLAYER_run_item2" {
	"anims/pm0740_00_fi21_run01.smd" 
	activity "ACT_MP_RUN_ITEM2" 1  	
	fps 60
	loop
}

$sequence "PLAYER_run_loser" {
	"anims/pm0740_00_fi21_run01.smd" 
	activity "ACT_MP_RUN_LOSERSTATE" 1  	
	fps 60
	loop
}

It would be:

$sequence "PLAYER_run" {
	"anims/pm0740_00_fi21_run01.smd" 
	activity "ACT_MP_RUN_ITEM1" 1  	
        activity "ACT_MP_RUN_ITEM2" 1  	
	activity "ACT_MP_RUN_LOSERSTATE" 1  	
	activity "ACT_MP_RUN_MELEE" 1  	
	activity "ACT_MP_RUN_MELEE_ALLCLASS" 1
	activity "ACT_MP_RUN_SECONDARY" 1 
	activity "ACT_MP_RUN_PRIMARY" 1  	
	activity "ACT_MP_RUN_PDA" 1  	
	fps 60
	loop
}

No source file, test, or entry point is identified in the issue. Start by locating the parser or compiler handling $sequence and activity directives, then compare its behavior with the proposed single-sequence examples; done means multiple activity lines can share one sequence without requiring duplicate animation definitions.