Add reset hotkey as Ctrl-R

I'm not entirely sure, that this is how it's supposed to be added.
This requires editing the already existing hotkey.json file (shows path
in log, if it exists).
Should this also be added to hotkey.cpp?

src/libresrc/default_hotkey.json

@@ -337,6 +337,9 @@
 		"video/frame/prev/large" : [
 			"Alt-Left"
 		],
+		"video/pan_reset" : [
+			"Ctrl-R"
+		],
 		"video/tool/clip" : [
 			"H"
 		],

src/libresrc/osx/default_hotkey.json

@@ -347,6 +347,9 @@
 		"video/frame/prev/large" : [
 			"Alt-Left"
 		],
+		"video/pan_reset" : [
+			"Ctrl-R"
+		],
 		"video/tool/clip" : [
 			"H"
 		],