Add Substitute texts for Fast-forward and Pause in replays

[roossienb]

• Follow up for: [GEN][ZH] Implement REPLAY NOT FOUND and MAP NOT FOUND dialogs when attempting to load a Replay that was deleted or whose map is missing #992
• Follow up for: [GEN][ZH] Implement feature to Pause and Step Frames in Replay playback #969
• Follow up for: [GEN] Backport fast forward feature for replay playback #827

The fast-forward function for replays in Generals was backported from ZH and doesn't have the fast-forward on/off message configured by default. It shows the placeholder instead. Use the substitute system to show the English text.

In ZH and Generals, use the substitute system to add a display message when the replay is paused an unpaused. This provides the user clear feedback of the action it performed

To be discussed: In ZH and Generals, use the substitute system to add a display message when a step frame command is given in a replay. This provides the user clear feedback of the action it performed

[xezon]

I think we can also add a message for after Replay load that prints the available key mappings for Fast Forward, Pause and Step Frame. I think there already is one for Fast Forward.