Disambiguate uses of the Play button label

This extends to a few other button labels used for replay control. While
checking if commit 46dbbc06c9 was fit for
backporting to 1.14 I found out that "Play" was already in use
exclusively for one of the replay control buttons (which means, no, it
can't be backported). This makes the disambiguation markers absolutely
necessary.

To give a more practical example of why this is a big deal, in Spanish,
"Play" would be translated as "Jugar" in the context of the Campaigns
menu, and "Reproducir" in the context of a replay (or movie). The
Spanish translation in fact already uses the latter in both 1.14 and
master.

[ci skip]

data/gui/window/campaign_dialog.cfg

@@ -440,7 +440,7 @@
 												id = "ok"
 												definition = "really_large"
 
-												label = _ "Play"
+												label = _ "game^Play"
 											[/button]
 
 										[/column]

data/gui/window/campaign_difficulty.cfg

@@ -218,7 +218,7 @@
 								[button]
 									id = "ok"
 									definition = "default"
-									label = _ "Play"
+									label = _ "game^Play"
 								[/button]
 
 							[/column]

data/themes/_initial.cfg

@@ -425,7 +425,7 @@
                 type=image
                 image=button_square/button_square_30
                 overlay=icons/action/play_30
-                title= _ "Play"
+                title= _ "replay^Play"
                 tooltip= _ "continuous replay"
                 items=playreplay
                 rect="=+7,=+5,+30,+30"
@@ -440,7 +440,7 @@
                 type=image
                 image=button_square/button_square_30
                 overlay=icons/action/pause_30
-                title= _ "Stop"
+                title= _ "replay^Stop"
                 tooltip= _ "pause after current move"
                 items=stopreplay
                 rect="+4,=+0,+30,+30"
@@ -456,7 +456,7 @@
                 type=image
                 image=button_square/button_square_30
                 overlay=icons/action/stop_30
-                title= _ "Reset"
+                title= _ "replay^Reset"
                 tooltip= _ "reset to beginning"
                 items=resetreplay
                 #rect="+4,=,+23,="