Never mistake an entry edited in place for one we generated

A desktop entry that already lives in ~/.local/share/applications is the
user's own file, patched in place with the original kept aside. It used to
get the same X-MCA-Generated marker as a shadow copy, which made the next
scan skip it, fall back to the system entry, and overwrite the user's file
with a shadow - and then delete it on revert instead of restoring it.

The two cases now carry different markers. Only a shadow is skipped by the
scan and deleted when undoing; an entry patched in place stays in the scan
and is restored from its backup.

Also: resolve the applications screen's labels once instead of per row per
keypress, and drop code nothing calls.

src/lib/apply.sh

@@ -8,7 +8,6 @@
 # of the behaviour, it is the behaviour.
 
 MCA_CHANGES=0
-MCA_TOUCHED=()
 MCA_ROUTES=()
 
 # mca_route <kind> <id> <program>
@@ -63,7 +62,6 @@ mca_apply() {
 	local i id file prog kind route steam_done=0
 
 	MCA_CHANGES=0
-	MCA_TOUCHED=()
 	MCA_ROUTES=()
 	MCA_N_ON=0; MCA_N_OFF=0; MCA_N_UNKNOWN=0
 
@@ -117,7 +115,6 @@ mca_apply() {
 	fi
 
 	mca_state_write last_apply "$(date +%s)"
-	mca_state_write covered "${#MCA_TOUCHED[@]}"
 	return 0
 }
 
@@ -131,7 +128,6 @@ mca_revert() {
 		update-desktop-database "$MCA_APPDIR" 2>/dev/null || true
 	fi
 
-	mca_state_write covered 0
 	return 0
 }