Add Category column and filter into income along with the exports

CHANGELOG.md

@@ -73,6 +73,9 @@ Only this release needs the command line for the database update. Normal updates
 - Update page: **Force Update no longer resets to `origin/master` regardless of the branch you track.** On an install following any other branch it silently moved the files onto master and discarded the code that was actually running. It now resets to the branch in `$repo_branch`, and so does `update_cli.php --force_update`. Installs with no `$repo_branch` in `config.php` still get master, as before.
 - Update page: the database update is now offered using a proper version comparison. The old string comparison would have stopped offering updates once a version reached a two-digit part — `2.6.10` sorts below `2.6.9` as plain text.
 - Update page: Git output — commit subjects and error text — is escaped before it reaches the page, and the branch name is escaped before it reaches a shell command. The page also no longer runs a second `git fetch` just to read the error message from the first one.
+- Income: added a Category filter and a Category column. A revenue carries its own category and a payment inherits the one on the invoice it was paid against, so both are categorised. Only categories actually in use are listed. Category is searchable and sortable, and it carries through to the export.
+- Income: the Source column now shows what the money was for. A revenue's category was being displayed there, which meant the revenue description never appeared on the page at all even though it is captured on the add and edit forms. Category has its own column now, so both are visible.
+- Income: fixed a PHP warning on every page load without a date range, and the export's filter summary, which was never populated - a filtered PDF came out looking like a full export.
 - Cron: the nightly run is safe to repeat. Late fees, overdue invoice reminders and autopay retries now apply at most once per invoice per day, so a Run Now after the scheduled pass no longer stacks fees or re-emails clients. Nightly Tasks only accepts the daily schedule.
 - Ticket SLAs, optional throughout. An SLA sets a response target and an optional resolution target, assigned per client and priority with a global default and an explicit "no SLA" override. Targets are measured against your business hours. Tickets show time remaining and turn yellow at a configurable warning threshold and red on breach, on both the ticket list and the kanban board, and can be filtered by SLA state. Nominated statuses pause the resolution clock for "waiting on customer", preserving the remaining budget. Two new reports, SLA Summary and SLA by Client. With no assignments defined nothing behaves any differently.
 - Ticket: added an Urgent priority.