[MC-108553] click events on signs with relative commands are inconsistent

Click events on signs are sometimes processed as the clicking player and sometimes as the sign itself.

To reproduce, put down a sign and put a command block on top of it with the respective command:

/blockdata ~ ~-1 ~ {Text2:"{\"text\":\"\",\"clickEvent\":{\"action\":\"run_command\",\"value\":\"[enter Command here, see below]\"}}"}

Activate the command block, right-click the sign.

Test-cases:

Command:
```
/say @p
```
This outputs "[name] name" of the clicking player, even if another player is closer to the sign.
Command:
```
/say @a[c=1]
```
Same as @p.
Command
```
/say @a[c=2]
```
Outputs the name of right-clicking player in square brackets but states the two closest players to the sign, in order of distance.
Command
```
/say @e[c=1]
```
Same as @p.
Command
```
/say @e[c=2]
```
Outputs the name of the clicking player in square brackets but states the two closest entities to the sign, in order of distance.
Command
```
/setblock ~ ~2 ~ gold_block
```
Sets a gold block 2 blocks above the sign.
Command
```
/teleport @p ~ ~2 ~
```
Teleports the player that clicked the sign 2 blocks above the sign.

These test-cases show an inconsistency in the processing of the commands on signs.