Implement listproj with factored out filtercommand() function.

_list() is way too large and monolithic for many (re-)use cases. As a first step, factor out the building of the filter_command and reuse that for the listproj filtering.
Enhance the listproj test with special cases that show the problems with the previous implementation directly using _list: Option -+, custom final filters, and non-ANSI colors cause it to break.

tests/t1260-listprj.sh

@@ -5,18 +5,37 @@ test_description='list project functionality
 . ./test-lib.sh
 
 cat > todo.txt <<EOF
-(B) smell the uppercase Roses +roses @outside
-(C) notice the sunflowers +sunflowers @garden
+(B) smell the uppercase Roses +roses @outside +shared
+(C) notice the sunflowers +sunflowers @garden +shared +landscape
 stop
 EOF
 test_todo_session 'basic listproj' <<EOF
 >>> todo.sh listproj
++landscape
 +roses
++shared
 +sunflowers
 EOF
 
 test_todo_session 'listproj with context' <<EOF
 >>> todo.sh listproj @garden
++landscape
++shared
++sunflowers
+EOF
+
+TEST_TODO_CUSTOM=todo-custom.cfg
+cat todo.cfg > "$TEST_TODO_CUSTOM"
+cat >> "$TEST_TODO_CUSTOM" <<'EOF'
+export DEFAULT='</color>'
+export PRI_B='<color type=green>'
+export PRI_C='<color type=blue>'
+export TODOTXT_FINAL_FILTER='grep -i roses'
+EOF
+test_todo_session 'listproj with context special cases' <<EOF
+>>> todo.sh -+ -d "$TEST_TODO_CUSTOM" listproj @garden
++landscape
++shared
 +sunflowers
 EOF